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.
What about replacing `explode` + `trim` by `preg_split('/\s*,\s*/', $l1, -1, PREG_SPLIT_NO_EMPTY)`? Thus, we avoid empty values.