Using a German male voice for speech synthesis.

See https://github.com/strands-project/memory_game/issues/47
Bug fixes: No actually uses the specified language.
Made the locale and voice more reconfigurable and usable for different kind of languages. Now able to switch between english and german voices via service call.
Added information on available voices and languages and instructions on how to switch to README.
Merge pull request #21 from cdondrup/german_male_voice

[ros_mary_tts] German voices