Expose DBH Classes and Entries in the admin

Polygons themselves are not included in the admin because testing showed
that they made the site too unstable to be of any real use.

Author: ahinz

polygons/admin.py

@@ -0,0 +1,5 @@
+from django.contrib.gis import admin
+from polygons.models import TreeRegionPolygon, DBHClass, TreeRegionEntry
+
+admin.site.register(DBHClass)
+admin.site.register(TreeRegionEntry)