Teacher can view student list (course show page)
Add Classroom object, refactor teachers course

By adding the Classroom object, we encapsulate the responsibility for
providing course views with the necessary objects.
Add Teacher controller specs
Add TeachersController to handle auth
Refactor controller authorization specs

Introduced shared example for controller authorization specs.

Attempted polymorphic implementation. This is a contrived example of
refactoring via polymorphism. Trying to follow the 'tell, don't ask'
principal could probably be done much better. Feedback welcomed.
Merge pull request #20 from iericallen/ea-refactor-controller-specs

Ea refactor controller specs