Extend PILEngine to allow other filters to work

[Muffinman]

Sorry Github auto-closed #19 when I renamed my branch to match yours.

Re-opening a new PR.

Fixes #12

[fdintino]

Sorry for the delayed response, I've been out on paternity leave.

I haven't had a chance to investigate, but in theory this shouldn't be necessary. The fact that it doesn't extend PILEngine is intentional; the __getattr__ method on this class is meant to dispatch to either the image engine or the video engine, depending on the circumstances. The PILEngine methods operate on an underlying static image, so its methods aren't applicable to videos, which are manipulated with calls to execute ffmpeg. In order to fully get these filters to work with the video engine, you would need to map the functionality to the corresponding ffmpeg command line arguments as methods on thumbor_video_engine.engines.video.Engine.