Add migration to set site domain and name.

Inspired by [this article][1], I automated the setting of the site domain and name (as mentioned in the README) by creating a data migration.

Not sure if that's the approach you were thinking of...but, it works!

Even though it doesn't really belong as a migration in the `users` app, that's where I put it, since it's the only place where you've got any migrations. I'm happy to move it to a better location as you see fit.

One idea would be to create a `core` app and store it in there, but it seemed like overkill to create a whole new app just to run a single migration.

Your thoughts?

[1]: http://codeinthehole.com/writing/a-data-migration-for-every-django-project/