Implement StreamingPull for consuming messages

[medikent]

StreamingPull could enable a more consistent message pull experience. Currently I am experiencing arbitrary pull delays in the order of seconds. Adding a StreamingPull implementation may address this. Either way, a StreamingPull implementation is a good step towards 1.0 of this library.

See Google PubSub's StreamingPull docs

PubSub was designed for high throughput and high message volumes and latency under low load has been experienced by many. See thisStackOverflow post and this Issue in the NodeJS pubsub library.