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/CSV/Sheet.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace Box\Spout\Reader\CSV;
+
+use Box\Spout\Reader\SheetInterface;
+
+/**
+ * Class Sheet
+ *
+ * @package Box\Spout\Reader\CSV
+ */
+class Sheet implements SheetInterface
+{
+    /** @var RowIterator To iterate over the CSV's rows */
+    protected $rowIterator;
+
+    /**
+     * @param resource $filePointer Pointer to the CSV file to read
+     * @param string $fieldDelimiter Character that delimits fields
+     * @param string $fieldEnclosure Character that enclose fields
+     * @param \Box\Spout\Common\Helper\GlobalFunctionsHelper $globalFunctionsHelper
+     */
+    public function __construct($filePointer, $fieldDelimiter, $fieldEnclosure, $globalFunctionsHelper)
+    {
+        $this->rowIterator = new RowIterator($filePointer, $fieldDelimiter, $fieldEnclosure, $globalFunctionsHelper);
+    }
+
+    /**
+     * @return RowIterator
+     */
+    public function getRowIterator()
+    {
+        return $this->rowIterator;
+    }
+}