Fixed #19463 -- Add UUIDField

Added UUIDField. Not particularly complicated this one - no custom lookups or anything. Casts all values to `uuid.UUID` instances in python.

TODO:
- [x] More form field tests
- [x] Documentation

Squashed patch should be marked as fixing #19463.
This requires `psycopg2>=2.0.9` but given that was [released in February 2009](https://github.com/psycopg/psycopg2/commit/71c7825b9cdca0ae62a1a5044928652aede3beb5) I don't think we need to worry about it.