Migrations

I think I've got an idea of how to do the migrations to different databases, but I need to make it a bit more concrete, specially about how to run those migrations in a production-like environment where there might be several schemas.

In in nutshell, the idea is that the migrations that affect the main schema inherit from one class, while migrations that affect the instance schemas need to inherit from another class. In this way, we can reuse the connection setup that we might use in the webapplication, plus we can try to write / extend a rake task that runs the migrations for all the schemas.