Db migrations

Implement a general mechanism for database migrations and change the script oq_create_db to use it.
See https://bugs.launchpad.net/oq-engine/+bug/1294566. Tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/543
Unfortunately, it looks like there is no Postgres syntax to inline comments. See for instance http://www.postgresql.org/docs/current/static/sql-createtable.html, there is no option to add a comment close to a column name as one would expect.