Expose higher-level Observable API

[dan]

We want to expose an API consumable by React 0.14 with the new observe hook for sideways data loading.

The observable we expose from DnD Core will register a drag source (or drop target) on subscribe, unregister it on unsubscribe and, given (context, backend) => data, return a stream of data that gets pushed on every context change event.

We will also probably try to separately build a higher-order component to polyfill observe until React 0.14 ships with it.

[dan]

We're now exposing subscribe(listener[, handlerIds]) : unsubscribe(). This is not really an observable but comes close, and we're not going to use observe hook anyway (yet?). We may revisit this after React 0.14.