Add config options for TTS engine (+ MaryTTS support)

This makes TTS engines have config options (e.g. `pitch_adjustment` or `words_per_minute` for Espeak TTS).

It also adds support for [MaryTTS](http://mary.dfki.de/). An online demo can be found [here](http://mary.dfki.de:59125/). I set `en_GB`/`dfki-spike` as default voice, because it actually makes you feel like you have a british butler. ;-)