Fix minor bugs and typos.

Fixes typos and invalid imports found in initial tests of original
code base. The first prevents Django from starting even from the
management shell. The second seems to be a hold-over error from using
the postgis module as a template and would cause problems during
introspectio.

 - The postgres creation module in the custom database backend
   imports from  django.db.backends.postgres.creation instead of
   what should be django.db.backends.postgres_psycopg2.creation

 - The postgres introspection module attempts to return a variable
   named "postgis_types" instead of the declared variable
   "rdkit_types"