assigns migration steps to User database objects

adds the migration_handler to the User db class, connecting the
migration steps in ../db/migrations.py to the migration code in
gmg_commands

mediagoblin/db/models.py

@@ -59,6 +59,8 @@ class User(Document):
         'verification_key': lambda: unicode(uuid.uuid4()),
         'is_admin': False}
         
+    migration_handler = migrations.UserMigration
+
     def check_login(self, password):
         """
         See if a user can login with this password