Shelf is activated now

* Shelf is activated with its features.

* Shelf can be created also shelf items can be added in particular shelf. 

* Relations are created for user with its shelves by RT "has_shelf"

* Now shelves can also be deleted. 
  Shelf deletion code added in ```ajax_views.py  -->  shelf()```

* Condition added for shelf items to add , only pages, files, quizes, courses and topic can add as a shelf items in user shelf.

* We wil be using this user shelf for building collection manager.