generator for Single Table Inherited models

I just add a ticket with a patch for a generator for STI models.

As explained in the ticket, if you want to generate a model which use STI mechanism, you have to use model generator with 2 options to avoid fixtures/migration creation and manually edit model file to set the right parent class.

I felt the need for a simple generator to generate this class, without having to add several options and modify generated code. The provided patch applies cleanly in master and 2-3-stable branches. Generator is provided with documentation and tests.

Ticket with patch: