Migration base class is not hard-coded anymore and it comes from the con...

This is implementation of issue #314. 

I needed to extend the migration base class, so I modified the Create command to inject the base class into the template as well. I've created a new config option named 'migration_base_class'. The default value is 'AbstractMigration'.
This line will remain, but I don't see how it could be modified, unless a full namespace is provided for `$baseClassName`, which I don't think would be advisable.