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...
The external dependencies of your components should always be injectable into your calls. Ideally, mandatory dependencies should be exposed via your constructor and optional dependencies should be exposed via publicly set-able properties. 
In this case, since the token database is a mandatory dependency, you should have a constructor overload where
I can pass the ITokenDatabase instance. This is the overload that your test cases would use where they will pass a mocked database instance.