Bump to 0.8.2

Make Button classes open to allow extensions

Added Swift 4 support

Add Carthage support, add FlatButton.selectedColor

FlatButton + PressableButton

Swift 3.0

Removed SwiftyButton.Style + made highlightedColor nullable

Added SwiftyButton.Style + SwiftyButton.highlightedColor

SwiftyCustomContentButton + title inset fix

Initial (simver) release