Secure problem

As described [here](https://www.reddit.com/r/golang/comments/2lu55c/restful_session_token/cly7mu6/), please change [your comparation](https://github.com/robbert229/jwt/blob/master/algorithms.go#L145) to default buildin [function](https://golang.org/pkg/crypto/hmac/#Equal) when hmac is used

With gratitude,
Vetcher