Database migrations

[edsu]

The next version of diffengine will require some database modifications for existing installs. peewee supports migrations. I think we have an example of one in init.py but maybe we should pull these out into a separate module?

I can test the migrations on a v0.2.7 database that I have.

[nahuelhds]

I was going to create an issue because I couldn't make to work the tweeting when using PostgreSql and it was because I had an older version of the Diff model schema and I needed to add the emailed column that was created at #77.

I lost a lof of debugging hours with this one :(

I say we create a /migrations/{version}/{engine}.sql file where the migration are written, so the user know that need to execute that one on the database he's using.