boss_db:migrate support in Mnesia

Hey Evan,

This provides migration support in MongoDB.

Migrations will fail with an exit if the migration table doesn't exist.  That means that, in addition to creating an "_id_" table, users need to create a "schema_migrations" table, with something like:
```erlang
mnesia:create_table('schema_migrations', [{disc_copies, Nodes}, {attributes, [id, version, migrated_at]}]).
```

Thanks!
-Lucas-