Don't load every migration all the time

Sometimes you reference Models in early migrations that have been renamed by the time you get to later versions.

This means your migrations will die with ugly errors :(

A fix is to only load the migrations that are going to actually run :)

This patch adds a MigrationStub that carries the name, version, and filename of a migration. That information is then used to load a migration when it's actually about to be run!

There are tests too :)

(For reference on it being a bug: http://groups.google.com/group/r...