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/downloads/struct.py

@@ -0,0 +1,21 @@
+import os
+import pickle
+from datetime import date
+
+from django.conf import settings
+
+from models import Product
+
+def get_struct():
+	# Products
+	for p in Product.objects.filter(approved=True):
+		yield ('download/products/%s/' % p.category_id,
+			   0.3)
+
+	# FTP browser
+	f = open(settings.FTP_PICKLE, "rb")
+	allnodes = pickle.load(f)
+	f.close()
+
+	for d in allnodes.keys():
+		yield ('ftp/%s' % d, None)