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/core/struct.py

@@ -0,0 +1,14 @@
+import os
+from datetime import datetime
+
+def get_struct():
+	yield ('', None)
+	yield ('community/', None)
+
+	# Enumerate all the templates that will generate pages
+	for root, dirs, files in os.walk('../templates/pages'):
+		r=root[19:] # Cut out ../templates/pages
+		for f in files:
+			if f.endswith('.html'):
+				yield (os.path.join(r, f),
+					   None)