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.
@staabm I understand @evert. This is a **helper**. So, basically, a helper has no design. This is just a collection of functions. Having one class per header would be the favourite design, I agree, but this is maybe overkill here? Don't know.