Implement DELETE operation on /process

It will call the cleanup method.
Remove eta and add output in serialization
Transcode: use html5 as default profile
Return output information from transcode operation
Use a shotdetect prefix for temp.dir
Return output information from shotdetect operation
Remove eta field
Implement progress as a property
Implement Process.serialize/to_json_string
Define custom states
Worker/producer decoupling

We cannot assume that workers have access to the DB. Hence, all worker operations must be independent from the DB.
Since completion callbacks are hard to get on the producer side, but subtasks are easy, the architectural choice is to have a local worker (AMQP queuename 'local'), which will be used to run completion methods that will update the DB.
Update INSTALL information
Update TODO
Add ignore_state parameter to cleanup_expired_processes method

It can be invoked by adding the force=1 parameter to the requested resource (either a single process or the list of processes)
Fix success_callback method

Properly update all fields
Add license header to source files