Django 1.7 compatibility fixes

* The first commit addresses #22.
* The second commit addresses system check warnings appearing on Django versions > 1.7.1 (https://github.com/django/django/commit/e9d1f1182aaccaa8b60cf6a3491f0103d2bb0229)
* The third commit allows -- if anyone likes -- choices to be passed in explicitly; before, the `choices` argument was explicitly there, but was immediately overwritten
* And lastly, instead of `__import__`ing things by hand, let's use Django's `import_by_path` method.