Add option to automatically call `refresh_from_db`

[jonashaag]

Let's say I'm lazy and specify timestamps as follows:

foo = mommy.make('Foo', created='2017-02-01')

Now foo.created == '2017-02-01'. This may be unhandy if you then want to use the value, for example in assertions:

self.assertEqual(foo.created, datetime.datetime(...))

[berinhard]

@jonashaag maybe we could add flag like mommy.make('Foo', created='2017-02-01', _refresh_after_create=True).

[berinhard]

Fixed by #364 .