Restore immovable value after Motion tweens and FlxPaths, closes #1279

+ less redundancy in FlxTween#finish()
FlxAssets#getFileReferences(): ingore invisible files, closes #1280
FlxButton: fix onUp for touches, closes #1278
Compile fix for targetting ealier swf versions
Add preStateCreation signal
FlxG.elapsed pass-through refactor
Fixed FlxObject.updateMotion, FlxEmitter and FlxVelocity
Merge branch 'dev' of http://www.github.com/HaxeFlixel/flixel into refactor_elapsed
removed nullable parameter in FlxVelocity
FlxObject: Removed unecessary variable in updateMotion
Merge pull request #1281 from larsiusprime/refactor_elapsed

Refactor elapsed, closes #1188
Merge.