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.
To elaborate on this, to correctly take advantage of the suggestion you are making, you are effectively proposing that I change this:

```php
class Something {

    protected $preferParser;

    function doSomething() {
        
        PreferParser::parse($str);

    }

}

$obj = new Something();
$obj->doSomething();

```

Into this:

```php

class Something {

    protected $preferParser;

    function doSomething() {

        $this->getPreferParser()->parse($str);

    }

    function setPreferParser(PreferParser $preferParser) {

        $this->preferParser = $preferParser;

    }
    function getPreferParser() {

        return $this->preferParser;

    }

}

$obj = new Something();
$obj->setPreferParser(new PreferParser());
$obj->doSomething();
```

With no practical use-case to do so.