Merge branch 'release/3.0.4' into develop

* release/3.0.4:
  Bump the version to 3.0.4.
Added RQ-based async cache file backend
Cache file backends: dropped the "Async" in class names
Merge pull request #258 from mkai/_rq_async_cachefile_backend

Added RQ-based async cache file backend
Fix celery backend
Correct reference to generateimage tag

Closes GH-261
Tagging 3.1.
Merge branch 'release/3.1'

* release/3.1:
  Tagging 3.1.
  Correct reference to generateimage tag
  Fix celery backend
  Cache file backends: dropped the "Async" in class names
  Added RQ-based async cache file backend