MIFOSX-982 - Add ability to associate a staff to user [WIP]

Details About Issue : https://mifosforge.jira.com/browse/MIFOSX-982

Work Done : 
1) Create user: Add an optional field staffId to associate existing staff to new user.
2) Get user: should also return the staff id along with other details if a staff is associated with it.
3) Update User: add ability for user to update user's linked staff 
4) Also Check if the Office Id of Staff is Same as that of the User, if not throw an exception. 

Work In Progress :
1) Create Staff: Add an optional field to associate existing user to new staff. 
2) Edit User : reference POST/user/2?command=associate&staffId=12
3) Edit User : POST/user/2?command=dissociate&staffId=12

Please review the Work Done So Far.