Simplify ThroughStream by using data callback instead of inheritance

[clue]

Previously, one had to extend the ThroughStream in order to implement custom filtering behavior. However, there is no public implementation of this available: http://packanalyst.com/class?q=React%5CStream%5CThroughStream

With this PR applied, one can now simply pass a callback function to its constructor like this:

$through = new ThroughStream(function ($data) {
    return json_encode($data) . PHP_EOL;
});
$through->on('data', $this->expectCallableOnceWith("[2, true]\n"));

$through->write(array(2, true));

Builds on top of #88