Revision support in history API
Flag query-based models accessed timestamp as uneditable

To be clear, this simply prevents it being exposed by default as an
editable form field in forms and the admin.
Update history subcommand to use cull manager method

Change default behavior to enable history since the overhead of saving
revisions are much more minimal than the previous implementation of
archiving instances.

The behavior of HISTORY_ENABLED now auto-registers the DataContext,
DataView and DataQuery models with the history API.
Add migration for Revision model
Merge pull request #127 from cbmi/history

Refactor history API