Added a DeprecationWarning if Async cache file backend is used
Removed unneeded django_rq import
setup.py: added 'async_rq' extra, django-rq dependency

django-rq v0.6.0 is the first version with Python 3 support.
Always call variable "source_file"

This emphasizes that it's a file object and not a PIL image and also connects it
to the earlier example which creates that variable.
Add note about `open()`

Hopefully this prevents people from going through acrobatics to get a File when
they've already got one!
Merge pull request #277 from mkai/_async_deprecation

Added a DeprecationWarning if (old) Async cache file backend is used
Merge pull request #278 from mkai/_rq_async_cleanup

Cleaned up RQ cache file backend code, added async_rq setup extra
Simplified RQ cache file backed by using the job decorator