Add `getSocket()` method to `ClientImpl` class

[alexmnv]

Getting the underlying socket might be necessary when PAMI is used as a part of a larger program.

For example, incorporating PAMI in ReactPHP EventLoop:

$loop = Factory::create();
$loop->addReadStream($PAMIClient->getSocket(), function($stream) use($PAMIClient) {
    $PAMIClient->process();
});

// ...

$loop->run();

[coveralls]

Coverage decreased (-0.04%) to 92.199% when pulling 78fd71b1f09b1b580afe048a5d56426b6b7a3ebc on alexmnv:patch-1 into f586d0fcbf7db7952965ccb91e4ed231bd168fb3 on marcelog:master.