adds nav bar update.
adds overrideBackButton option
adds if options.overrideBackButton? conditional.
adds toParams and getCallback to NavigationBarButton.
title is removed if image title is set.
updating buttons on one side no longer overrides previously-set buttons on the other side.
if not's replaced by existential operator.
title overrides titleImage.
little restructuring.
Merge pull request #17 from AppGyver/topic/navigationBar_update

Topic/navigationBar.update