Allow passing parameters for callback when forking.

This allows for cleaner code in someplaces where you would otherwise have to use a closure, which can be hard to test.

My use case.

I have a class with a method where i want to fork out, that method takes one argument like

``` php
<?php

function doSomethingExciting($myLuckyNumber) {
    // do something here
}
```
 Currently i would have to do something like

``` php
<?

$lucky = 42;

$manager = new Spork\ProcessManager;
$manager->fork(function ($fifo) use ($lucky) {
    doSomethingExiting($lucky);
});
```

Instead of

```
<?php

function doSomethingExciting($fifo, $lucky) {
}

$manager = new Sprok\ProcessManager;
$manager->fork('doSomethingExciting', $lucky);
```