You need tox to run all the tests
Add a more discoverable way of running all the tests

It's more characters, but gets people used to using our
Makefile.
Version bump, in anticipation of future release

This release is technically backwards incompatible, in that
it fixes quite a few bugs. There'll be a changelog in the
upcoming documentation.
Add a test for saving an empty recurrence object

I wasn't sure whether this would fail on models which don't
accept null values. Turns out it's allowed, so we should
make sure it stays allowed.
Add some basic docs (#23)