Simple Mongo -> SQL migration tool

Simple Mongo -> SQL migration tool

This is just a start at a Migration tool from Mongo to SQL.
It fills all currently available SQL models with data from
MongoDB. A few fields in the SQL tables are left out,
because some data format migrations are needed (notably:
queue_file_name).

This thing lives in mediagoblin/db/sql/convert.py because
it has a lot of stuff hardcoded and is not, repeat not for
end users!

Hard coded:
- output database: ./mediagoblin.db (sqlite)
- Mediagoblin config: ./mediagoblin.ini