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 agree with both of you. Its more or less a academic point.

Having a helper like this is most of the time bad design itself. Which rules apply for helper classes do not matter, because you should avoid them ;-).

Helper classes tend to get even bigger because every logic which doesn't find in a "regular class" tend to land in those helper class sooner or later.