Add Multi-User Authentication

[thieman]

Spun off from #5, see original discussion there.

This issue concerns adding a multi-user auth model to Dagobah. This would add various user-level permissions to the core Dagobah classes, such as user-level ownership and sharing of a specific Job. The exact changes we want to make as a result of this change are open to discussion.

This will almost certainly be built off of the work done to close single-user auth, #11.