Fixed #2443 -- Add DurationField

This has been added to core as well following design decision taken in https://code.djangoproject.com/ticket/2443

This field was never merged due to UI related issues. Personally I think a CharField with `DD HH:MM:SS.uuuuuu` formatting in it is sufficient - this is basically what a `TimeField` does anyway.

Uses `bigint` of microseconds for non-psql databases, `interval` on postgres.

Todo:
- [x] docs
- [x] form field
Thanks for clarifying. It would be nice to make the test more specific in some way (maybe backend specific), but if you feel that's too much work, please at least include this explanation in the test.