OPAL-1878: User and Group classes and service
OPAL-1879: Implemented user services
OPAL-1880: Implemeneted group web services
OPAL-1879: Check if users is null and test case
OPAL-1882: User and Group python commands
OPAL-1878: Shiro integration. Created a new realm: OpalRealm.
OPAL-1879: Services form singular and plural forms (/users, /user/{name} and /groups, /group/{name})