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.