This allows all models inherited from PgModel to specify which
URLs to purge by either setting a field or defining a function
called purge_urls, at which point they will be purged whenever
the save signal is fired.
Also implements a form under /admin/purge/ that allows for manual
purging. This should probably be extended in the future to show
the status of the pgq slaves, but that will come later.
Includes a SQL function that posts the expires to a pgq queue. For
a local deployment, this can be replaced with a simple void function
to turn off varnish purging.
Each module now contains a struct.py file that will return all
the URLs that it can generate (yes, this is a small break of the
abstraction of url.py, but we've broken that elsewhere as well),
and also which search-engine-weight (0.1-1.0) that this URL should
be given.
