Fix error when adding custom fields generators to bakery via settings file

[berinhard]

Fixes #18

The many comments on #18 describes why this work was required, but the short summary is that the instruction in the docs to set generators for custom fields via settings.py wasn't working. Django raises the exception AppRegistryNotReady if the code tries to import anything related to the DB before properly creating the models.

Model bakery wasn't respecting this rule because it was importing the ContentType model and a few django.contrib.gis models too. This PR fixes this by replacing direct imports by module's imports and by moving others imports to be done during the execution time.

[berinhard]

@amureki and @anapaulagomes this is a very old bug that I finally sat down to fix and, thanks to jah, it was waaaaay more trivial than the issue suggested ¬¬. Whenever you have time to review it'd be nice! I think we can release a new version after this PR is merged.

[anapaulagomes]

Wow, @berinhard is on fire! 🔥

I only miss a test for it. I wonder if would be possible to test it adding custom fields to a settings file since we don't have one in the current setup (I'm on vacation, so I've skipped testing any approach locally, sorry :P)

[berinhard]

@anapaulagomes thanks for the test remember! I added it in 1ff384a

Do you feel we can merge this? If so, I'll also release a new bakery's version.

[anapaulagomes]

A tiny thing is left to change. Also, Travis must be green before merging it. :)

[berinhard]

Great! Once the build finishes and passes, I'll merge this 👍