Token service priti

Implemented Basic Token Service.. Wrote Test cases for the same.
Further extensions can be done to set session timeout period and accordingly maintain sessions. But not sure whether session timeout period is responsibility of TokenService or Log In module...
It might make some sense to include a code understandable field to identify the type of error.
E.g., errorcode or type. In most cases, clients would use this field to identify the category of the fault and decide
on what to do next. All the other fields are non deterministic in terms of their content.