Proposal: Add support for observable spec interop point

[benlesh]

Per the stage 1 observable proposal, there is an interop point that can be added for types that are "observable". It's as simple as adding a [Symbol.observable]() hook that returns a basic observable (it doesn't have to be Rx-based or anything).

Pros

1. Adding this hook will enable interoperability with RxJS 5, and it should do the same for Kefir, Bacon, Most, and others.
2. People can stop asking me "what's the difference between RxJS and redux?"
3. People that want to use a more reactive methodology for delivering state to components can do so without forgoing the wonderful tooling this community has put together
4. It's a relatively minimal change

   Cons

Need to include a small Symbol.observable shim. Should probably add some integration test to a common library which adds one development-time dependency.

attn/ @gaearon @zenparsing

[gaearon]

Looks 👍 to me!

[xirzec]

This dependency seems to break using Redux with TypeScript and webpack: https://github.com/TypeStrong/ts-loader/issues/192