Parses multiple documents in a stream

Thank you for publishing this gem! It's the closest replacement to yajl-ruby for me when I'm on JRuby In my use case I needed to be able to parse multiple JSON documents from a stream. This patch allows `JSON::Stream::Parser` to parse one or more sequential documents. I wasn't really sure what to do with `JSON::Stream::Builder` (I'm not using it) so I tested the existing behavior, which simply uses the last document in the stream. Let me know if you're interested in merging this and if you need anything changed first.

Cheers!