Downloads: celery broker and backend added

Some restructuring to downloading view to process post request from downloads
Asynchronous tasks on broker (amqp) now should work (if it's even needed...)
Downloads: Archiving and Status tracking

See #8
Working: Archiving iteratively now works, updates status per file zipped
Updated: Views (+ corresponding serializer), celery and tasks
To implement: JQuery + Ajax to update template with status bar, serve file
tasks - reorganised

slight reorganization of code to have state updated first