User model introduced

Renamed student in many places to user
student is now an STI model, same as teachers

Commented out teachers spec (for now)
Merge branch 'master' into feature/45-users-sti

Conflicts:
	app/controllers/application_controller.rb
	app/controllers/sessions_controller.rb
	spec/controllers/sessions_controller_spec.rb
	spec/features/authentication_spec.rb
current_student -> current_user
Merge pull request #23 from lighthouse-labs/feature/45-users-sti

User model introduced