Closes: #505.

This PR exposes the Model POSet to the plugins so they can use complete class hierarchy information to determine if the analyzed class is a descendant of another class.

Consider this example:

class ApplicationController; end
class MyAbstractController < ApplicationController; end
class MyController < MyAbstractController; end

Before this change, a plugin working on MyController didn't have any global knowledge about the codebase and couldn't see farther than the direct superclass of MyAbstractController, this led use to use some Regex hacking on the class name like filtering classes names based on /Controller$/.

With Model#symbols_hierarchy we can finally get the full ancestry and see that MyController indeed descends from ApplicationController.