Job returning None as result are now persisted correctly.
Job status can now be checked via ``status`` property which should
return either "queued", "finished" or "failed".
Merge pull request #119 from selwin/worker-connection

Worker.find_by_key now properly uses connection. Thanks.
I like this implementation of an 'enum' better.
Add convenience accessor properties for status.
Don't use strings, but comments.
Add expiry info to the worker log.
Remove is_done property.

It is too similar to is_finished.
Merge branch 'selwin-persist-None-result'
Update changelog.
Fetch fresh status value on every request.

It could probably require a bit of caching, to prevent too many fetches
per time slot (for example, return the locally cached value if that
value is as fresh as a second or so).

This fixes #120.