Fix iOS v8 runtime crashing

[Archez]

fixes #83

This should fix issue #83 when running with an iOS v8 runtime >7.0.6 and/or with latest nativescript/core. I couldn't figure it out exactly why, but the objc marshaller was crashing with a bad pointer access when trying to wrap the native object params for the button handler methods. Since the native event param was not being used, simply removing it was sufficient to fix the crash.

I also rearranged the button handler implementation/declaration to be more in line with the rest of the nativescript core components (which should hopefully improve garbage collection in theory).