Extracts VoiceNavigationController to encapsulate TTS logic.
Replace US with U.S.

The Android TTS engine speaks the pattern "US" as "United States".

This commit replaces "US" with "U.S." in spoken instructions which forces the TTS engine to say the initials only.
Ignores semi-colon, colon, and comma in spoken instructions.
Merge pull request #244 from mapzen/chuck/voice-nav-improvements

Voice Navigation Improvements