Multi curl

[innerspirit]

Adds support for doing post/put/delete requests in parallel

Example:

$meli = new Meli($appId, $appKey);
$meli->put('items/'.$item_1, $body_1, $params_1, true); // true = enqueue, defaults to false
$meli->put('items/'.$item_1, $body_2, $params_2, true);

$responses = $meli->executeQueued();
// Both puts are executed at the same time

Note: this requires #52 and #53

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This change is