Simpler SQS support

This implements support for Amazon SQS in a simple way with the `AmazonSqsQueue` class.

You need the `aws/aws-sdk` package installed in at least version `2.1`.

Example:

```php
<?php

use Kue\AmazonSqsQueue;
use Aws\Common\Enum\Region;

$queue = new AmazonSqsQueue("my-queue", array(
  'key' => '…',
  'secret' => '…',
  'region' => Region::US_EAST_1
));

$queue->push($job);
```

The `AmazonSqsQueue` has also a special `->deleteJob()` method which takes a `Job` instance. This can be hooked up to the worker's `success` event to automatically delete finished jobs from the queue.

```php
<?php

use Kue\Command\WorkCommand;
use Kue\Job;

$app = new \Symfony\Component\Console\Application('example');

$work = new WorkCommand($queue, array(
  'success' => function(Job $job) use ($queue) {
    $queue->deleteJob($job);
  }
));

$app->add($work);

$app->run()
```