[#42] Batch publishing

This is to address https://github.com/polleverywhere/firehose/issues/42

@bradgessler or @steel I'd appreciate another pair of eyes on this before it goes into master. If you are happy with this, then I'd like to use this to take a stab at implementing a client-side (publisher-side) version of https://github.com/polleverywhere/firehose/issues/35 (configurable buffer sizes).