Added basic icon support to FlxUIButton.

Using these methods:

addIcon( icon:FlxSprite )
removeIcon()
changeIcon(newIcon:FlxSprite)
removing isStatic constructor argument for text classes
Merge branch 'master' of https://github.com/HaxeFlixel/flixel-ui
Fixed bug which made checkboxes and radio buttons in a tab menu all clicked by default.