Make -currentValueForControlStateKey: public

Useful for subclasses
Merge branch 'btrpopupbutton-alignment' into btrcontrol-state-additions
Merge branch 'master' into btrcontrol-state-additions
Add constants for state keys
Update commment
Merge pull request #48 from ButterKit/btrcontrol-state-additions

Make -currentValueForControlStateKey: public
Accessibility support for BTRPopUpButton
Accessibility support for BTRActivityIndicator
Accessibility support for BTRImageView
Accessibility support for BTRControl
Accessibility support for BTRButton
More accessibility for BTRControl
Add accessibility actions support for BTRButton and BTRPopUpButton
Style
Merge remote-tracking branch 'origin/accessibility' into clean-up