Add missing method to App Engine memcache driver
Always import assets on App Engine

When running on GAE, assets are precompiled so we can't use the
in-memory copy since any assets requiring compilation (e.g. less
or coffescript) will fail to build due to GAE restrictions.
Add a couple of debug statements in datastore queries

Allows the user to see the queries that are being executed
Always fail when a datastore field can't be mapped with the model

Previously, we allowed unmapped fields and let the datastore sort
it out, but it caused silent errors difficult to find and fix.