Fix #112 move static constants on path following from FlxObject to FlxPath

The old "PATH_" prefix no longer makes sense, so it was dropped, e.g. FlxObject.PATH_FORWARD is now FlxPath.FORWARD.
Add deprecate warnings in FlxObject.
Merge pull request #218 from Dovyski/fix-issue-112

Move static constants on path following from FlxObject to FlxPath.