Callback update

Hi.

I think this callback will be better. Why:
1) I want get callback always for detect finish of checking. For example I set flag "executing=YES" before call "detectAppVersion:" and I set flag "executing=NO" inside callback. It's flag for check that "detectAppVersion:" already running.
2) I add BOOL return value for callback. It allow you show default dialog after callback execution (return YES) or create your own dialog (return NO). 

Please check this and release new version to cocoapods. Thx.