Moved readers to iterators

Instead of the hasNext() / next() syntax, readers now implements the PHP iterator pattern.
It allows readers to be used with a foreach() loop.

All readers now share the same structure (CSV is treated as having exactly one sheet):
- one concrete Reader
- one SheetIterator, exposed by the Reader
- one or more Sheets, returned at every iteration
- one RowIterator, exposed by the Sheet

Introducing the concept of sheets for CSV may be kind of confusing but it makes Spout way more consistent.
Also, this confusion may be resolved by creating a wrapper around the readers if needed.

-- This commit does not delete the old files, not change the folder structure for Writers. This will be done in another commit.

tests/Spout/Reader/XLSX/Helper/SharedStringsHelperTest.php

@@ -0,0 +1,112 @@
+<?php
+
+namespace Box\Spout\Reader\XLSX\Helper;
+
+use Box\Spout\Reader\XLSX\Helper\SharedStringsCaching\CachingStrategyFactory;
+use Box\Spout\Reader\XLSX\Helper\SharedStringsCaching\FileBasedStrategy;
+use Box\Spout\Reader\XLSX\Helper\SharedStringsCaching\InMemoryStrategy;
+use Box\Spout\TestUsingResource;
+
+/**
+ * Class SharedStringsHelperTest
+ *
+ * @package Box\Spout\Reader\XLSX\Helper
+ */
+class SharedStringsHelperTest extends \PHPUnit_Framework_TestCase
+{
+    use TestUsingResource;
+
+    /** @var SharedStringsHelper */
+    private $sharedStringsHelper;
+
+    /**
+     * @return void
+     */
+    public function setUp()
+    {
+        $resourcePath = $this->getResourcePath('one_sheet_with_shared_strings.xlsx');
+        $this->sharedStringsHelper = new SharedStringsHelper($resourcePath);
+    }
+
+    /**
+     * @return void
+     */
+    public function tearDown()
+    {
+        $this->sharedStringsHelper->cleanup();
+    }
+
+    /**
+     * @expectedException \Box\Spout\Reader\Exception\SharedStringNotFoundException
+     * @return void
+     */
+    public function testGetStringAtIndexShouldThrowExceptionIfStringNotFound()
+    {
+        $this->sharedStringsHelper->extractSharedStrings();
+        $this->sharedStringsHelper->getStringAtIndex(PHP_INT_MAX);
+    }
+
+    /**
+     * @return void
+     */
+    public function testGetStringAtIndexShouldReturnTheCorrectStringIfFound()
+    {
+        $this->sharedStringsHelper->extractSharedStrings();
+
+        $sharedString = $this->sharedStringsHelper->getStringAtIndex(0);
+        $this->assertEquals('s1--A1', $sharedString);
+
+        $sharedString = $this->sharedStringsHelper->getStringAtIndex(24);
+        $this->assertEquals('s1--E5', $sharedString);
+
+        $usedCachingStrategy = \ReflectionHelper::getValueOnObject($this->sharedStringsHelper, 'cachingStrategy');
+        $this->assertTrue($usedCachingStrategy instanceof InMemoryStrategy);
+    }
+
+    /**
+     * @return void
+     */
+    public function testGetStringAtIndexShouldWorkWithMultilineStrings()
+    {
+        $resourcePath = $this->getResourcePath('one_sheet_with_shared_multiline_strings.xlsx');
+        $sharedStringsHelper = new SharedStringsHelper($resourcePath);
+
+        $sharedStringsHelper->extractSharedStrings();
+
+        $sharedString = $sharedStringsHelper->getStringAtIndex(0);
+        $this->assertEquals("s1\nA1", $sharedString);
+
+        $sharedString = $sharedStringsHelper->getStringAtIndex(24);
+        $this->assertEquals("s1\nE5", $sharedString);
+
+        $sharedStringsHelper->cleanup();
+    }
+
+    /**
+     * @return void
+     */
+    public function testGetStringAtIndexWithFileBasedStrategy()
+    {
+        // force the file-based strategy by setting no memory limit
+        $originalMemoryLimit = ini_get('memory_limit');
+        ini_set('memory_limit', '-1');
+
+        $resourcePath = $this->getResourcePath('sheet_with_lots_of_shared_strings.xlsx');
+        $sharedStringsHelper = new SharedStringsHelper($resourcePath);
+
+        $sharedStringsHelper->extractSharedStrings();
+
+        $sharedString = $sharedStringsHelper->getStringAtIndex(0);
+        $this->assertEquals('str', $sharedString);
+
+        $sharedString = $sharedStringsHelper->getStringAtIndex(CachingStrategyFactory::MAX_NUM_STRINGS_PER_TEMP_FILE + 1);
+        $this->assertEquals('str', $sharedString);
+
+        $usedCachingStrategy = \ReflectionHelper::getValueOnObject($sharedStringsHelper, 'cachingStrategy');
+        $this->assertTrue($usedCachingStrategy instanceof FileBasedStrategy);
+
+        $sharedStringsHelper->cleanup();
+
+        ini_set('memory_limit', $originalMemoryLimit);
+    }
+}