Added 'active' attribute to FlxButton

Based of findings here: https://github.com/HaxeFlixel/flixel/pull/1227
Would allow setting "active" for an FlxButton in the XML. Helpful if you want to prevent a login button from being pressed until the fields are filled. 
Also updated the documentation (added active and visible in the button attributes list).
Only thing speaking against it is there's currently nothing in place for it... yet.