Implement UsersController#create.

Now saves users to database.  Redirects to root on success, which does
not yet have a template.
Stub home page.

Creating a stub template for the 'application#index' route.
Test valid registration form.

Confirm that new user is created.  Requires that we pull in the
database_cleaner gem -- because Selenium runs in a separate process,
the standard transactional rollback that rspec uses was causing the
database to lock and fail the test.