Temporal/Revisions Model

Re-submitting the temporal/revisions model.

This one has had various bug fixes and extra useful features added since the original PR.

If this is to be merged in perhaps it's worth mentioning on the fuelphp website as (as far as I can see) no other ORM currently supports temporal/revisions so would make a good selling point.

Docs:
https://github.com/fuel/docs/pull/493