Implement jti for JWT Bearer tokens to prevent replay attacks.

Closes #265.

In https://github.com/bshaffer/oauth2-server-php/issues/265#issuecomment-29544814 I mentioned that there is some uncertainty as to how "unique" we need our "jti" matching to be. 

With my implementation, that is now the responsibility of the library user. He decides how he wants to return the `jti` in the `getJti()` storage method.