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'.
The two `str_replace` commands could be combined using arrays, but this is okay too.