Add pubsub#subscription, pubsub#subscribe and pubsub#getSubscriptions

[ryanseys]

Add pubsub#subscription and pubsub#subscribe and pubsub#getSubscriptions.

All the logic I feel should move to these requests and then topic#subscription and topic#subscribe and topic#getSubscriptions should either be removed or rely on the pubsub version with an optional topic field automatically passed in.

How's this sound @stephenplusplus?

Reference: #487

/cc @tmatsuo

[jgeewax]

Can we clarify the use case on this ? #487 is long, so it'd be nice if the description said "why" we're doing this.

I have one outstanding question, which if answered, makes me OK with these new pubsub-level methods. I also would agree that if we do add these, the topic-level ones simply wrap the pubsub-level ones. That is:

Topic.prototype.subscribe = function(...) {
  return this.path.to.pubsubInstance.subscribe(...);
};

[ryanseys]

The use case is for creating a subscription in one project that subscribes to a topic in another project.

[stephenplusplus]

@ryanseys SGTM. In fact, I can start working on this now, but feel free to stop me if you already had this in the works.

[ryanseys]

@stephenplusplus You can go for it!