Properly handle an external language like plv8. Fixes #97.

  * pyrseas/database.py (Database._link_refs): Change query against
    pg_pltemplate to join of pg_language and pg_depend.
    (Database.diff_map): Move extensions._drop further down.
  * pyrseas/dbobject/extension.py (ExtensionDict.diff_map): Mark
    extensions for dropping.