adds pubsub for veneer, updates pubsub app and README

[bshaffer]

No description provided.

[tmatsuo]

Now it should be service instead of module

[tmatsuo]

How about to use Google\Cloud\PubSub\PubSubClient like:

use Google\Cloud\PubSub\PubSubClient;
$pubSub = new PubSubClient([
    'projectId' => 'my_project'
]);

// Get an instance of a previously created topic.
$topic = $pubSub->topic('my_topic');

// Publish a message to the topic.
$topic->publish([
    'data' => 'My new message.',
    'attributes' => [
        'location' => 'Detroit'
    ]
]);

instead of using Google\Cloud\ServiceBuilder?