Allow for the retrieval of unverified header values

[mark-adams]

@skion commented on #70 that it might be useful to have a way to parse out the unverified header parameters in the library prior to validating the entire JWT so that, for instance, a key could be looked up using the kid (until we get JWK support completed in 2.0)

We probably should create a jwt.utils function called get_unverified_header() or something like that.