Properly Handle UTF-8 Encoded Characters

Based on the [PHPWord CodePlex](http://phpword.codeplex.com/discussions/261365) discussion boards from 2011 there is an issue regarding proper handeling of UTF-8 encoded strings. The issue seems to arise from an already encoded UTF-8 string being sent through the `utf8_encode` method. 

Rather than just remove the `utf8_encode` method as proposed on the discussion board. My patch detects if the string is already UTF-8 encoded `mb_detect_encoding($text) === 'UTF-8'`. If the string is already properly encoded no additional steps are taken.

As far as I can test/tell, it is safe to remove the function call all together. However, there could be some edge case that I cannot see where this maybe needed.

*Also included is an example case that outputs proper UTF-8 encoded strings.*