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 generally do understand dependency injection, but I guess I simply hold the opinion it's not a 'one size fits all' solution. API > implementation for me :) 

@Hywan I agree that there is a possibility that at some point in the future, state is required. But how it's required and how it will then be used may not affect the original use-case, and without knowing in advance how we'll need to use it, we may need to refactor in the future later anyway.

At that point the logical decision might still be to simply pass a second argument ;)