Removing duplicate test
Don't raise ValueError for saving None into null=False (fixes #22)

Thanks to @mbertheau for the report (and patch) - this
change just amends the tests to reflect the new behaviour.
Get the codebase into a state of reasonable PEP 8 compliance

We now test this with tox (you can check it with make
testall, or tox -e flake8).

We've skipped over a few rules where I think they make code
less clear - though fixes are welcome if you can find ways
to remove the exceptions (in tox.ini) and keep the code
clean.
Let's ensure tox builds the documentation too

This way it's easy to test that the documentation is in a
good state, and tox can take care of installing
documentation dependencies for you.
Adding tox tests for more combinations of Python/Django versions
Splitting up the documentation

Hopefully I'll be expanding the documentation shortly, so
splitting it up should make that easier to do.
Add a bit more detail for how to use RecurrenceField