Adding expiration support, replaced jwt-simple with node-jsonwebtoken, 

I've changed the package's JWT library from [jwt-simple](https://github.com/hokaccha/node-jwt-simple) to [node-jsonwebtoken](https://github.com/auth0/node-jsonwebtoken). In doing so, I was able to add an error check on expired tokens, so a client can handle the expiration properly, i.e. request a new token.

My reasoning for the change is jwt-simple currently does not implement token expiration, [nor is it planned to be added in the near future](https://github.com/hokaccha/node-jwt-simple/issues/8). node-jsonwebtoken is also a much more robust library, and is being actively maintained.

 This change is not breaking.