Cleanup for flixel.addons

This includes my previous pull request #484, so please merge them in correct order.

#395

- renamed `FlxAngle.DEG` to `FlxAngle.TO_DEG`
- renamed `FlxAngle.RAD` to `FlxAngle.TO_RAD`
- renamed `BTNTilemap` to `FlxRayCastTilemap`
- renamed `OgmoLevelLoader` to `FlxOgmoLoader`
- created a new package `flixel.addons.plugin` for the `AntTaskManager`