Support .get(fieldname) on sql db objects

Some parts of the code like to call .get("somefield") on
the db objects.  It's easy to support this on sqlalchemy
based objects, so lets do it.

mediagoblin/db/sql/base.py

@@ -25,3 +25,6 @@ class GMGTableBase(object):
     @classmethod
     def one(cls, query_dict):
         return cls.find(query_dict).one()
+
+    def get(self, key):
+        return getattr(self, key)