Updating "active_fedora:model" rails generator

The `rails generate active_fedora:model CLASS_NAME` command now accepts
additional arguements for customizing the generated active fedora
object.

@WIP - It may be nice to wire this in as a model so we could use
scaffold and resource generators from Rails. Much like rspec allows you
to hook into the models for generation and FactoryGirl allows you to
override the generation of fixtures.

Closes #91