[add:php] Add Users model, controller and view

Add the Users model, controller and the view for managing Authentication
of the system.
[mod:php] Modify Admin, Staff and Student Models

Modify Administrator, Staff and Student models to relefect the
changes in the database (adding the users table) an new associations.
[add:modaca_db.sql] Add the modified database dump

Add the modified database dump that contains the changes to the database.
Merge branch 'master' of https://github.com/kasunkv/MoDACA