[contrib.postgres] 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
Not much difference either way I guess. The `ArrayField` tests were committed with it this way. `.create()` doesn't really do any magic so I can change them all if you like.