Allow swappable Observable implementation

[mAAdhaTTah]

This is long-future stuff, but any-observable provides a way for a library author to depend on a generic Observable and allow the consumer to choose which Observable implementation they want to use. Right now, we have a hard dependency on Kefir, and are returning Kefir.Observables for all the library methods. Allowing the library consumers to choose which Observable implementation will allow brookjs to be more flexible. Especially given the popularity of RxJS, I suspect consumers are going to want to use that, although I've found the complexity of dealing with the various builds hard to deal with and much prefer the smaller surface area and semantics of Kefir.

Major Blockers:

• [ ] es-observable spec ratification
  • Spec
  • Current spec also doesn't include most of the useful methods (map, flatMap, etc.)
• [ ] Kefir spec conformity
  • This issue is likely to make things difficult, as it conflicts w/ Kefir's semantics and is opposed by Kefir's developer.

[mAAdhaTTah]

We're not going to be able to do this, as I don't think Kefir will ever conform to the ESObservable spec. We can revisit this at some point in the future.