Interface iterations

[rgbkrk]

• [x] Technically the iopub should not be a subject. It should only be an Observable. Still have to rely on onCompleted, still a subject.
• [x] I totally cheated and declared subj.send = subj.onNext to fit our intent rather than RxJS's parlance. I like this better so people can write shell.send(payload). Somebody once told me that adding props at runtime is a performance drag. Once I see that being an issue, we can adapt it. Now we're using RxJS 5 / ES7 Observable parlance.
• [x] Since these get tied together as subjects, the user is required to subscribe to any they want to onNext ahem send on. Ok, this is still an issue. Sweep it under the rug!

[rgbkrk]

As for 1, we do need to be able to dispose of the underlying socket, via onCompleted. It does make sense to be a subject then. It just shouldn't try to send messages across. Tempting to make subj.close = subj.onCompleted, just like subj.send = subj.onNext. Carries the intent better, which is my answer to (2).

For (3), I'm wondering if there is a better approach to handling these from Rx. I technically want to be able to use the Observer without making a subscription to the Observable.