Add support for generating sitemap.

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.

pgweb/featurematrix/struct.py

@@ -0,0 +1,4 @@
+from datetime import date
+
+def get_struct():
+	yield ('about/featurematrix/', None)