handle tweets split over chunk boundaries

The Twitter streaming API recently changed to split tweets over several chunks, which was causing JSON parsing exceptions (see bemoaning here: https://groups.google.com/d/topic/clojure/dLMjY2i9tCg/discussion). 

New to Clojure so feedback enthusiastically welcome!

this change is on the advice of @adamwynne, as tweets can have null chars https://github.com/adamwynne/twitter-api/issues/47#issuecomment-41583078