Site's city (or lack thereof) can now be changed on-the-fly.

directory.urls no longer looks at the site object to determine
what URL paths are valid; instead, it registers everything, and
individual routes determine at time of access whether to 404 or not.

We also clear the Site cache whenever City objects are modified so that
such changes take place instantaneously. This avoids lots of potential
confusion.
always force SITE_ID=1 by default when running test suite.
added tests for the city_scoped decorator.