Include executed statement in /sessions/:id/statements return

Based on this user list chain (https://groups.google.com/a/cloudera.org/forum/#!topic/livy-user/UNEUp1RAiSE) there is interest in having the original executed statement returned with its output. With the addition of the Web UI this is a good potential addition to the REST API and UI. From a quick look at plumbing this seems a non-trivial task since the original statement is not currently saved.