Basic SQS integration

[selul]

I've added a basic SQS integration, following the Rabbitmq example.

It requires defining an $aws_credentials array having this format:

global $aws_credentials;
$aws_credentials = array(
	'region'      => 'us-east-1',
	'version'     => 'latest',
	'credentials' => array(
		'key'    => '', //AWS key.
		'secret' => ' ' //AWS secret.
	)
);

The queue name is hardcoded to wordpress. The PR implements the system tests also, but it will require to define in config.php file the $aws_credentials array.