Added keep alive functionality

I have added the keepAlive functionality. Now when the server has no activity since 10 minutes or more the TeamSpeak 3 server don't close the telnet session.

I use threading module to do that, plugins are not affected by this feature
Update ts3bot.py

I forgot to use the variable self.keepAliveInterval
Merge pull request #4 from Antoine-Pous/patch-2

Added keep alive functionality