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.
here we go.

either the parser is something you like to be inject (because you have different use-cases) it should be passed in using the constructor (no setter/getter).

```
class Something {
    protected $preferParser;

    public function __construct($parser) {
       $this->preferParser = $parser;
    }

    function doSomething() {
        $this->preferParser->parse($str);
    }
}

$obj = new Something(new PreferParser());
$obj->doSomething();
```

if you don't need separate parsers you could of course also use a default and no constructor arg


```
class Something {
    protected $preferParser;

    public function __construct() {
       $this->preferParser = new PreferParser();
    }

    function doSomething() {
        $this->preferParser->parse($str);
    }
}

$obj = new Something();
$obj->doSomething();
```

In a bigger system/subsystem where `Something` would be a class which is part of your public API you might even use a depency injection container[1], so the user will get a `Something` instance from it. In this case the parser would also be injected via `__construct` but this would be done by the container and it will be hidden for the api consumer

```
class Something {
    protected $preferParser;

    public function __construct($parser) {
       $this->preferParser = $parser;
    }

    function doSomething() {
        $this->preferParser->parse($str);
    }
}

// bootstrap of your app
$injector = new Auryn\Provider;
$injector->share(new PreferParser()); // this assumes you use 1 prefer-parser for all your classes created by this injector (so for your subsystem)

// api consumer
// ... the consumer needs to somehow get access to the injector
$somethingObj = $injector->make('Something); 
// the injector made sure that all required deps (constructor args) are passed in, and that e.g. a second class which uses this parser would get the very same parser
$somethingObj->doSomething();

// of course it would also be possible to instruct the injector to create a new Parser for each class which requires one.
```

[1] https://github.com/rdlowrey/Auryn