Implementing CourseCart service

[kyukyukyu]

• [x] Adding courses to course cart
  • [x] Add single course object
  • [x] Broadcast addtocart event
  • [x] Set course group as required when major course object is added
  • [x] Set course group as optional when general course object is added
  • [x] Group course objects by their subjects when added
  • [x] Add course objects with same code only once
• [x] Removing courses from course cart
  • [x] Remove single course object
  • [x] Broadcast removefromcart event
  • [x] Remove course group if no course is in it
• [x] Broadcast changerequiredincart event when a required course group is changed to optional one or vice versa
• [x] Clear course cart when selected campus is changed