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.
For my point of view, we can remove the static part by passing an instance of the `HeaderHelper` wherever we need it:
```php
$headerHelper = new HeaderHelper();
$httpParser = new Parser($headerHelper);
```
Why I would go for removing static declaration? Because at a certain time, I am sure that parsing a specific HTTP header will require the state/value of another HTTP header somewhere else. Thus, it's not going to be stateless. And static is —by nature— stateless.

If all methods are statics, then simple named functions can do the job.