Use list of languages in pg_pltemplate in Database._link_refs. Fixes #103.

 * pyrseas/database.py: If running in PG 9.1+, select the list of
   pg_pltemplate languages and pass it to the languages link_refs
   method.
 * pyrseas/dbobject/language.py: Use the list of languages and not just
   plpgsql.
 * tests/dbobject/test_function.py: New test, courtesy of DVarrazzo.