Wagtail 2 support

[alanmoo]

Closes #8

I'm not super familiar with writing Django plugins but this seems to run for me locally in Wagtail 2.0. One file that may still need updating is test_helpers.py, I'm not sure what the best practice is for writing tests against multiple versions of an app.

cc @chosak

[chosak]

Thanks very much for contributing this @alanmoo! I've incorporated your changes into #11, which also adjusts the unit tests to test against these new versions, and makes a few other minor related changes. If you have a chance, would you mind testing out that branch?