Page-node can now also be deleted (actually hiding from UI) but only by it's creator


* In models.py file:
  - STATUS_CHOICES_TU: Variable added that holds mongokit's IS operator with values (u'DRAFT', u'HIDDEN', u'PUBLISHED')
  - This variable is assigned to Node class's 'status' field; it means 'status' can hold a value only from one of the three given values!
  - By default, for status field -- 'DRAFT' value is set from now onwards for every document that gets created.
  
* In urls/page.py file:
  - 'page_delete' url is added that accepts group_name and node's ObjectId as it's parameters; and in turn calls to a view function named 'delete_page'.
  
* In views/page.py file:
  - 'delete_page' view function: Updates the status field to 'HIDDEN' for the node whose ObjectId is passed to it! 
    NOTE: Here instead of mongodb's save() function, update() function is used.
  - In page-list-view, those pages are listed: which are public pages, belongs to the current accessed group and whose status is other than 'HIDDEN'.
  
* In node_edit_base.html:
  - Positions of Tag and Privacy elements are swapped.