Several improvements

I made several little tweaks to the field. There were a few subtle bugs introduced in recent commits, which should be fixed. I also gave the test suite some love. Two features were added; specificing ``PHONENUMBER_DEFAULT_FORMAT`` (instead of defaulting to E164), and hinting at the region when using ``parse_string``.

I've added a note on ``null=True`` being discouraged for usage with the ``PhoneNumberField``. As with ``CharFields``, in most cases one doesn't want two different "empty" statuses, ``''`` and ``None``. 

I suggest making ``PhoneNumberField`` stricter, only accepting valid phone numbers by default. But that's a backwards-incompatible change and should be discussed.