Add OnLongClick Injection

Personally, I've run into wanting OnLongClick injection lately, so I figured I would go ahead and add it to the library.

I updated the ViewId class to be able to support additional MethodInjection types (via a Set, like you are doing for Views). I'm a little on the fence about using the Enum to determine the type, I just didn't want to have to pass the class object, a string, or something else around.

One weird thing to note (I have comments to show it as well): The equals method that I added to MethodInjection only compares the objects (null/correct type/etc.) and the new MethodInjectionType. I figure that given anything that ButterKnife might inject to handle a view that it would only ever want one method being injected for that single type at any given time.

That way a single view could have both an OnClick and OnLongClick annotation, since that is valid.