Send fork-result back to parent on wait()

@fauvel @tomaszdurka I wanted to use process forking for faster image processing on the webserver.
Turns out that's not a good idea within php-fpm.
Nevertheless I think something along these lines might be useful (for CLI).
Basically when a process' fork()-workload has finished, I send back the return-value through a stream
to the parent process. Within that result I also contain the exception, if one was thrown. This allows
to process exceptions from forked processes.

What do you think?

Example usage:
```php
$process = CM_Process::getInstance();

for ($i = 0; $i < 3; $i++) {
    $process->fork(function () use ($i) {
        for ($t = 0; $t < 3; $t++) {
            if ($t == 1 && $i == 0) {
                throw new Exception('jo');
            }
            sleep(1);
            echo "$i - $t\n";
        }
        return 'foo';
    });
}

$resultList = $process->waitForChildren();
var_dump($resultList);
```

Output:
```
0 - 0
2 - 0
1 - 0
Exception: jo in /home/vagrant/fuboo/scripts/test.php on line 16
    0. {main} /home/vagrant/fuboo/scripts/test.php:0
    1. CM_Process->fork(Closure) /home/vagrant/fuboo/scripts/test.php:22
    2. CM_Process_ForkHandler->runAndSendWorkload() /home/vagrant/CM/library/CM/Process.php:48
    3. {closure}() /home/vagrant/CM/library/CM/Process/ForkHandler.php:46

2 - 1
1 - 1
2 - 2
1 - 2
array(3) {
  [0] =>
  class CM_Process_WorkloadResult#208 (2) {
    private $_result =>
    NULL
    private $_exception =>
    class CM_ExceptionHandling_SerializableException#207 (7) {
      public $class =>
      string(9) "Exception"
      public $message =>
      string(2) "jo"
      public $line =>
      int(16)
      public $file =>
      string(36) "/home/vagrant/fuboo/scripts/test.php"
      public $trace =>
      array(4) {
        ...
      }
      public $traceString =>
      NULL
      public $metaInfo =>
      array(0) {
        ...
      }
    }
  }
  [1] =>
  class CM_Process_WorkloadResult#213 (2) {
    private $_result =>
    string(3) "foo"
    private $_exception =>
    NULL
  }
  [2] =>
  class CM_Process_WorkloadResult#209 (2) {
    private $_result =>
    string(3) "foo"
    private $_exception =>
    NULL
  }
}
```