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.

src/Spout/Reader/XLSX/Reader.php

@@ -0,0 +1,93 @@
+<?php
+
+namespace Box\Spout\Reader\XLSX;
+
+use Box\Spout\Common\Exception\IOException;
+use Box\Spout\Reader\AbstractReader2;
+use Box\Spout\Reader\XLSX\Helper\SharedStringsHelper;
+
+/**
+ * Class Reader
+ * This class provides support to read data from a XLSX file
+ *
+ * @package Box\Spout\Reader\XLSX
+ */
+class Reader extends AbstractReader2
+{
+    /** @var string Temporary folder where the temporary files will be created */
+    protected $tempFolder;
+
+    /** @var \ZipArchive */
+    protected $zip;
+
+    /** @var \Box\Spout\Reader\XLSX\Helper\SharedStringsHelper Helper to work with shared strings */
+    protected $sharedStringsHelper;
+
+    /** @var SheetIterator To iterator over the XLSX sheets */
+    protected $sheetIterator;
+
+
+    /**
+     * @param string $tempFolder Temporary folder where the temporary files will be created
+     * @return Reader
+     */
+    public function setTempFolder($tempFolder)
+    {
+        $this->tempFolder = $tempFolder;
+        return $this;
+    }
+
+    /**
+     * Opens the file at the given file path to make it ready to be read.
+     * It also parses the sharedStrings.xml file to get all the shared strings available in memory
+     * and fetches all the available sheets.
+     *
+     * @param  string $filePath Path of the file to be read
+     * @return void
+     * @throws \Box\Spout\Common\Exception\IOException If the file at the given path or its content cannot be read
+     * @throws \Box\Spout\Reader\Exception\NoSheetsFoundException If there are no sheets in the file
+     */
+    protected function openReader($filePath)
+    {
+        $this->zip = new \ZipArchive();
+
+        if ($this->zip->open($filePath) === true) {
+            $this->sharedStringsHelper = new SharedStringsHelper($filePath, $this->tempFolder);
+
+            if ($this->sharedStringsHelper->hasSharedStrings()) {
+                // Extracts all the strings from the sheets for easy access in the future
+                $this->sharedStringsHelper->extractSharedStrings();
+            }
+
+            $this->sheetIterator = new SheetIterator($filePath, $this->sharedStringsHelper, $this->globalFunctionsHelper);
+        } else {
+            throw new IOException('Could not open ' . $filePath . ' for reading.');
+        }
+    }
+
+    /**
+     * Returns an iterator to iterate over sheets.
+     *
+     * @return SheetIterator To iterate over sheets
+     */
+    public function getSheetIterator()
+    {
+        return $this->sheetIterator;
+    }
+
+    /**
+     * Closes the reader. To be used after reading the file.
+     *
+     * @return void
+     */
+    protected function closeReader()
+    {
+        if ($this->zip) {
+            $this->zip->close();
+        }
+
+        if ($this->sharedStringsHelper) {
+            $this->sharedStringsHelper->cleanup();
+        }
+    }
+}