Support for german

Hey,
this pull-request adds support for german (`GermanTextTranslator`).

I wanted to use `AbstractTextTranslator` at first (like the others do), but german is quite a complicate language (compared to others) and after finding myself on overwriting all the methods the abstract class has, i decided to just implement the `TextTranslatorInterface` and throw the abstract class away. I tried to copy the coding-style of the abstract class tho.

I also extended the specs for the new language.


At last, i need to thank you for this package. Seems like this is the only one, which supports multiple languages and is not bound to a big framework. I also like the implementation of the translators as classes, otherwise it would maybe not be possible to implement such languages like german. :)