Added BubbleWrap.use_weak_callbacks, default to false.
When set to true, #weak! will be used for UIControlWrapper.

Also: https://github.com/rubymotion/BubbleWrap/issues/322#issuecomment-30558903
Call #weak! to fix leak with UIViewWrapper various forms of #when_XXX
Tested BubbleWrap.use_weak_callbacks=true
fix test for #when_tapped working with BubbleWrap.use_weak_callbacks=true
Merge pull request #324 from hboon/call_weak_on_callbacks

Added BubbleWrap.use_weak_callbacks, default to false.