Detection of invalid sheet name - continued

Invalid names can also be triggered by:
- character ":"
- single quote at the beginning of the name
- single quote at the end of the name

Introduced a StringHelper, wrapping multibyte strings functions

tests/Spout/Writer/XLSX/SheetTest.php

@@ -65,8 +65,11 @@ class SheetTest extends \PHPUnit_Framework_TestCase
             ['Illegal /'],
             ['Illegal ?'],
             ['Illegal *'],
+            ['Illegal :'],
             ['Illegal ['],
             ['Illegal ]'],
+            ['\'Illegal start'],
+            ['Illegal end\''],
         ];
     }