Support other UUID generators in auto_add.

This commit adds the ability to specify UUIDField.auto_add as a
callable, rather than just boolean True. If a callable is specified, it
is called with no arguments.

This allows for:

    id = models.UUIDField(auto_add=uuid.uuid1, primary_key=True)

...or similar flexibility.
Merge branch 'master' into uuid-flexible-auto-add-unstable
Merge pull request #7 from lukesneeringer/uuid-flexible-auto-add-unstable

Support other UUID generators in auto_add.