Add a check for db schema version

nipapd should refuse to start if the PostgreSQL db schema version is
outdated.

The up-to-date schema version (required to run) is stored inside
nipap/__init__.py
The current schema version is read out of the database comment.

The web-ui ( /version ) will also show the schema version.
Merge pull request #488 from dosomder/460-check-db-version

Add a check for db schema version