Implement basic varnish purging

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.

pgweb/urls.py

@@ -108,6 +108,7 @@ urlpatterns = patterns('',
 
 	# Override some URLs in admin, to provide our own pages
 	(r'^admin/pending/$', 'pgweb.core.views.admin_pending'),
+	(r'^admin/purge/$', 'pgweb.core.views.admin_purge'),
     # Uncomment the next line to enable the admin:
     (r'^admin/(.*)', admin.site.root),