update to use the AMD module version of FactoryGuy

I've just created danielspaniel/ember-data-factory-guy#38, which creates an AMD module out of FactoryGuy that can be used like any other ember-cli module.

This pull request depends on that one, so it won't work until FactoryGuy 0.8.4 is released. It updates ember-cli-data-factory-guy to use the new AMD formatted module, and adds instructions to the README on how to use the module version.

Thanks!