[stdlib] Add bridge from Varying to various other common abstractions

[issa-tseng]

eg: Varying.asPromise(v).succeedIf(f).failureIf(f) -> Promise

Promises, Rx Observable, anything else that's popular and semantically harmonious enough can be considered. Probably rely on peerDependencies or dependency injection to keep our own dependencies at a minimum.

As detailed in this discussion: https://github.com/socrata-platform/janus/pull/2#discussion_r90362349

[issa-tseng]

janus-stdlib now provides fromEvent(jqInstance, eventName, f) which gives a Varying where given an f: (event, …) -> value will map event firings into values for the Varying. It's implemented via Varying.managed, which means the resource lifecycle of the event listener is correctly handled and nothing happens unless the Varying is actually reacted against.