A new tutorial about migrations

@Avenirer, there's a small error in this line of code (part of the undo_migration() method in the Migrate controller):

PHP Code:

$penultimate = (sizeof($migration_keys)==1) ? 0 : $migration_keys[sizeof(migration_keys) - 2]; 


The $ sign before migration_keys in sizeof(migration_keys) is missing. PHP says it's an undefined constant.
Other than that, it works great!!