Basic, extremely naive authentication API.

This duplicates, but not quite, much of `cas_provider.views.login`. Only email/password combinations are supported. No socialauth, no complicated authentication workflows. No, not even merges.
Add an appropriate error for the merge scenario, for now.
Allow signal handlers to run.
Fixed error message for incorrect email/password combo.
Merge pull request #2 from Crossway/feature/auth-api

Basic, extremely naive authentication API.