1- Made some changes to task table of the database and made corresponding
   changes to the code.
2- Added table for category to the database and added code for getting
   categories from the server and displaying them. Also implemented
   adding new categories in place.
3- Implemented the tree(hierarchical)display for categories, along with
   expand/collapse functionality.
Merge branch 'master' of https://github.com/rakhound/task-management