topicName format is inconsistent

[gbowman85]

Hi Spencer.

I'm new to Pub/Sub so your library has been really helpful.

In the documentation it's not clear what is required for topicName when using SubscriptionApp.newSubscription(subscriptionName,topicName,webhookUrl).

I assumed it was the same requirements as with newTopic but it is the full topic name of projects/{project}/topics/{topic}

It feels a bit inconsistent that the library handles the simple topic name when creating a topic but not when creating a subscription.