Expose a Sheet object on Reader::XLSX::nextSheet()

Added Sheet class for the XLSX reader that exposes basic sheet info, such as name or ID.
When retrieving the sheet data XML, added extra XML parsing to retrieve sheet data.
Added test

tests/Spout/Reader/XLSXTest.php

@@ -200,6 +200,32 @@ class XLSXTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals([], $allRows, 'Sheet with no cells should be correctly processed.');
     }
 
+    /**
+     * @return void
+     */
+    public function testNextSheetShouldReturnCorrectSheetInfos()
+    {
+        $resourcePath = $this->getResourcePath('two_sheets_with_custom_names.xlsx');
+        $reader = ReaderFactory::create(Type::XLSX);
+        $reader->open($resourcePath);
+
+        /** @var \Box\Spout\Reader\Sheet[] $sheets */
+        $sheets = [];
+        while ($reader->hasNextSheet()) {
+            $sheets[] = $reader->nextSheet();
+        }
+
+        $reader->close();
+
+        $this->assertEquals('CustomName1', $sheets[0]->getName());
+        $this->assertEquals(0, $sheets[0]->getNumber());
+        $this->assertEquals(1, $sheets[0]->getId());
+
+        $this->assertEquals('CustomName2', $sheets[1]->getName());
+        $this->assertEquals(1, $sheets[1]->getNumber());
+        $this->assertEquals(2, $sheets[1]->getId());
+    }
+
     /**
      * @param string $fileName
      * @return array All the read rows the given file