Creating a new HeaderHelper

This PR moves functions from the Util class to the HeaderHelper and deprecates the old functions.

It also introduces a new parser for the `Prefer:` header, which unfortunately is still incomplete, as [rfc7240](http://tools.ietf.org/html/rfc7240) has a number of bugs.
That's a very general statement, and I don't feel it applies to every case.

I think this is an obvious case where instantiating the object and calling the method would virtually always happen in the exact same spot.

It would be theoretically possible to not do this, and allow people to inject this parser, but I think practically speaking this is not really realistic and will mostly result in an inconvenience.