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.
Well... making these methods static does not prevent anyone from calling them non-static:

```php

$o = new HeaderHelper();
$o->parseHttpDate($foo);

```

Is perfectly valid... but the main question I'd ask, is why would anyone do this?