Provide a way to specify TO/FROM languages using String instead of Language Enum

[boatmeme]

Need to provide a way to use the API without having to use the Language Enum. Preferably, user should be able to pass in a String representing the Microsoft Translator Language code.

This would allow the Java API to support new languages as the Microsoft service adds support, without the need to release new code (i.e., a new instance of the Language Enum).

[nicdnb]

Could probably use GetLanguagesForTranslate Method
and also GetLanguagesForSpeak Method