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/SheetIterator.php

@@ -0,0 +1,112 @@
+<?php
+
+namespace Box\Spout\Reader\XLSX;
+
+use Box\Spout\Reader\IteratorInterface;
+use Box\Spout\Reader\XLSX\Helper\SheetHelper;
+use Box\Spout\Reader\Exception\NoSheetsFoundException;
+
+/**
+ * Class SheetIterator
+ * Iterate over XLSX sheet.
+ *
+ * @package Box\Spout\Reader\XLSX
+ */
+class SheetIterator implements IteratorInterface
+{
+    /** @var Sheet[] The list of sheet present in the file */
+    protected $sheets;
+
+    /** @var int The index of the sheet being read (zero-based) */
+    protected $currentSheetIndex;
+
+    /**
+     * @param string $filePath Path of the file to be read
+     * @param \Box\Spout\Reader\XLSX\Helper\SharedStringsHelper $sharedStringsHelper
+     * @param \Box\Spout\Common\Helper\GlobalFunctionsHelper $globalFunctionsHelper
+     * @throws \Box\Spout\Reader\Exception\NoSheetsFoundException If there are no sheets in the file
+     */
+    public function __construct($filePath, $sharedStringsHelper, $globalFunctionsHelper)
+    {
+        // Fetch all available sheets
+        $sheetHelper = new SheetHelper($filePath, $sharedStringsHelper, $globalFunctionsHelper);
+        $this->sheets = $sheetHelper->getSheets();
+
+        if (count($this->sheets) === 0) {
+            throw new NoSheetsFoundException('The file must contain at least one sheet.');
+        }
+    }
+
+    /**
+     * Rewind the Iterator to the first element
+     * @link http://php.net/manual/en/iterator.rewind.php
+     *
+     * @return void
+     */
+    public function rewind()
+    {
+        $this->currentSheetIndex = 0;
+    }
+
+    /**
+     * Checks if current position is valid
+     * @link http://php.net/manual/en/iterator.valid.php
+     *
+     * @return boolean
+     */
+    public function valid()
+    {
+        return ($this->currentSheetIndex < count($this->sheets));
+    }
+
+    /**
+     * Move forward to next element
+     * @link http://php.net/manual/en/iterator.next.php
+     *
+     * @return void
+     */
+    public function next()
+    {
+        if (array_key_exists($this->currentSheetIndex, $this->sheets)) {
+            $currentSheet = $this->sheets[$this->currentSheetIndex];
+            $currentSheet->getRowIterator()->end();
+
+            $this->currentSheetIndex++;
+        }
+    }
+
+    /**
+     * Return the current element
+     * @link http://php.net/manual/en/iterator.current.php
+     *
+     * @return Sheet
+     */
+    public function current()
+    {
+        return $this->sheets[$this->currentSheetIndex];
+    }
+
+    /**
+     * Return the key of the current element
+     * @link http://php.net/manual/en/iterator.key.php
+     *
+     * @return int
+     */
+    public function key()
+    {
+        return $this->currentSheetIndex + 1;
+    }
+
+    /**
+     * Cleans up what was created to iterate over the object.
+     *
+     * @return void
+     */
+    public function end()
+    {
+        // make sure we are not leaking memory in case the iteration stopped before the end
+        foreach ($this->sheets as $sheet) {
+            $sheet->getRowIterator()->end();
+        }
+    }
+}