Support for Lazy assertions / assertions chains

[Taluu]

As the assert from beberlei's package propose them, so we can build assertions :

<?php

$assert = Assert::that($value);
$assert->not();
$assert->same('foo');

This is the global idea.

[BackEndTea]

I'd love to see this for this package, but i'm not sure how to go about that without a BC break. If you can create a PR that would allow this functionality, or allow for an easy upgrade path then id like it.

[arjanfrans]

I'd like to see this in this package too. I actually have an implementation for this. I'll see if it can be easily integrated and make a merge request.