Engine inclusion

This is an augmented version of #61.  The method for getting all the engines changes between Rails 3 and 4, so this supports both.  This feature is really awesome, so I would love to see it mainlined.  Our particular use case is several applications sharing a core data model, and I want to be able to graph it from the Rails application, since the data model knows about nothing more than `active_support`, while `railroady` requires rails.

I have been a fan of Railroad for several years now - thanks for taking up the maintenance of it!
Shouldn't this be more like `"#{engine.root.to_s}/app/models/**/*.rb"` ?