Avoid importing the whole project

When trying to install the project with pip we get an error as `setup.py` is calling `pgcrypto.__init__.py` which import both `base.py` (for the version) and `fields.py` for shortcuts.
Since `fields.py` is importing django this is producing a `django.core.exceptions.ImproperlyConfigured`.

I think it would be better to remove `from .fields import *` which is wrapped in `try..except` which ignore the error. And to remove in the `__init__` from base as they are reimported in `fields.py`