Add schema load initialization

Hi, thanks for this gem!

I'd like to get your feedback on a couple of things, and hope to get this pull-request merged in, if possible.

1.  According to the commit history, the rails dependency on 3.1.0 was due to the location of `IrreversibleMigration`, but when I checked, that class has not moved from 3.0.0 to 4.1.0.  In the PR, I've moved it back to 3.0.10

2. I added some code to populate the data_migrations table on `db:schema:load` to avoid running data migration code in a system that should not need it.  This is similar to how schema migrations don't run after a `db:schema:load` because the schema_migrations is pre-populated with all the migrations up to the version specified in the schema.rb.

Thanks!