Add alternative American Audio VMS2 controller mapping.

The mapping for VMS2 that comes with Mixxx lacks in some places.
This is an alternative mapping that also supports the rotational
knob for browsing that is not found on the VMS4.
Also secondary loop controls (previously unused) are mapped to
kill switches and some other minor improvements.
Apply some cleanup to VMS2 that is also proposed for VMS4.

Builtin Play and Cue handlers can be used instead of script binding.
We still keep the pause handler for the dedicated pause button.
More cleanup in VMS2 scripts. Remove now unused variables.
Merge pull request #163 from snue/feature/aavms2

Alternative mapping for American Audio VMS2