add question & answer model
Adding tag model
Adding Course Invitation
Merge branch 'almgohar'

Conflicts:
	app/models/course.rb
	app/models/user.rb
Merge branch 'master' into bassem

Conflicts:
	app/models/question.rb
	app/models/user.rb
Adding Rating super model
Database Migration
Fixing announcmenet type column
Creating Rating STI