Add a stub transcoder for development and testing purposes.

I do not have zencoder or ffmpeg transcoding set up in every environment where site development is taking place.

For his purpose I have created a stub transcoder which can be used to emulate a working ffmpeg transcoder.

• It can create thumbnails for videos just like devel_generate.
• It is very strict in checking parameters to catch faults sooner.
• It can log extra debug information to watchdog.
• It has a configurable delay to emulate real transcoding and test timeouts.

When it comes to transcoding it will simply copy the original video to a new file. Of course these videos will probably not be usable but everything else should work just as if you had a real transcoder set up.