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.
With static methods you couple your callsites to the methods. Also its not possible/harder to subclass/extend (this point is not perfectly valid for php, because of late static binding).