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/Sheet.php

@@ -0,0 +1,74 @@
+<?php
+
+namespace Box\Spout\Reader\XLSX;
+
+use Box\Spout\Reader\SheetInterface;
+
+/**
+ * Class Sheet
+ * Represents a sheet within a XLSX file
+ *
+ * @package Box\Spout\Reader\XLSX
+ */
+class Sheet implements SheetInterface
+{
+    /** @var RowIterator To iterate over sheet's rows */
+    protected $rowIterator;
+
+    /** @var int ID of the sheet */
+    protected $id;
+
+    /** @var int Index of the sheet, based on order of creation (zero-based) */
+    protected $index;
+
+    /** @var string Name of the sheet */
+    protected $name;
+
+    /**
+     * @param string $filePath Path of the XLSX file being read
+     * @param string $sheetDataXMLFilePath Path of the sheet data XML file as in [Content_Types].xml
+     * @param Helper\SharedStringsHelper Helper to work with shared strings
+     * @param int $sheetId ID of the sheet
+     * @param int $sheetIndex Index of the sheet, based on order of creation (zero-based)
+     * @param string $sheetName Name of the sheet
+     */
+    function __construct($filePath, $sheetDataXMLFilePath, $sharedStringsHelper, $sheetId, $sheetIndex, $sheetName)
+    {
+        $this->rowIterator = new RowIterator($filePath, $sheetDataXMLFilePath, $sharedStringsHelper);
+        $this->id = $sheetId;
+        $this->index = $sheetIndex;
+        $this->name = $sheetName;
+    }
+
+    /**
+     * @return RowIterator
+     */
+    public function getRowIterator()
+    {
+        return $this->rowIterator;
+    }
+
+    /**
+     * @return int ID of the sheet
+     */
+    public function getId()
+    {
+        return $this->id;
+    }
+
+    /**
+     * @return int Index of the sheet, based on order of creation (zero-based)
+     */
+    public function getIndex()
+    {
+        return $this->index;
+    }
+
+    /**
+     * @return string Name of the sheet
+     */
+    public function getName()
+    {
+        return $this->name;
+    }
+}