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.
I went from `Util` to `HeaderHelper` to make it more descriptive and actually makes them a bit more related.

If I were to put every header parser into its own class, I feel that the main thing that I'd gain, is simply a lot more classes. Also, I don't really see what I would gain by forcing people to instantiate this type of thing. Polymorphism isn't really relevant here.