Better KeyWord Spotting and Wit result management

Reactivated auto-threshold in the vader
Better management of Wit results :
- Client speaks differently when asking to repeat or not
- When no results from Wit, check if there was no start (only silence) and do not ask for repeat
- check confidence on Wit result (configuration param), and don't transmit to the server

Although first commit message tells that, the speak at login is not removed