Fix #104: MPbutton redefined

I know `MP` stands for `MixPanel`, but apparently it can also stand for `MediaPlayer`. Short of renaming everything to `MixPanelSomething` I renamed the known conflict to `MPActionButton`. Happy to do a full rename of everything `MP` to `MixPanel` if you would like.

Updated podspec with the correct version, ARC, and minimum platform. This pod doesn't build without ARC.