Custom parameter support in all supported gateways.

Adding support to send custom parameters.

send() method changed to send(array $datas = array(), $doMerge = true) to accept custom parameters.

use Omnipay\Common\GatewayFactory;

$gateway = GatewayFactory::create($gatewayName);
$gateway->initialize($gatewaySettings);

$defaultParams = array();
$request = $gateway->authorize($defaultParams);

$params = $request->getData();
$params['custom'] = 'value';

$response = $request->send($params, false);