CDAP-531: CLI (common) - Need to have inbuilt Help and Exit commands

These changes do not affect the current COOPR, CDAP CLIs, but need to remove Exit and Help commands from other CLIs. I will do this after this PR will be merged. Here also added QuitCommand due to https://issues.cask.co/browse/COOPR-623. @wolf31o2 please take a look. I have tested this changes on COOPR CLI.
Oh, will 
> help help 
work?