Convert tests to specs

There are both tests and specs in the application. In disucussion with @tkwidmer and @backspace we settled on preferring to use specs over tests.

To complete:
- [x] Update README to indicate that new code should be covered with specs.

And convert the following tests to specs:

- [x] test/controllers/bathrooms_controller_test.rb
- [x] test/controllers/contacts_controller_test.rb
- [x] test/controllers/welcome_controller_test.rb
- [x] test/geocoder_test_helper.rb
- [x] test/helpers/bathrooms_helper_test.rb
- [x] test/helpers/contacts_helper_test.rb
- [x] test/helpers/welcome_helper_test.rb
- [x] test/models/admin_user_test.rb
- [x] test/models/bathroom_test.rb
- [x] test/models/rating_level_test.rb
- [x] test/test_helper.rb

Also,

- [x] Configure Rails to not generate tests. (possibly autogenerating specs)