Feature: Distribute work

[thekid]

This PR implements #4 and allows distributing elements from a sequence on workers by using Sequence::distribute():

$workers= [
  new LocalWorkerProcess('com.example.parallel.ImageResizer', [800, 600]),
  new LocalWorkerProcess('com.example.parallel.ImageResizer', [800, 600])
];

Sequence::of(new IOCollectionIterator($dir))
  ->filter(new ExtensionEqualsFilter('.jpg'))
  ->map(function($e) { return $e->getUri(); })
  ->distribute(new WorkerProcesses($workers))
  ->each('util.cmd.Console::writeLine')
;

foreach ($workers as $worker) {
  $worker->shutdown();
}