Move refetch(), startPolling() and stopPolling() from Subscription to ObservableQuery

Interoperability with other Observable implementations (see #149) means we have no control over the Subscription object users end up with.

For example, Rx.Observable.from(handle).subscribe() would return an RxJS Subscription object without the Apollo-specific helper methods.

So I think it makes most sense to move these methods to ObservableQuery (maybe this needs a better name, like QueryHandle or simply Query).

This will require some refactoring of the current code to share fetching and polling logic between observers, but that actually seems like a better design anyway.

const handle = queryManager.watchQuery({ query });
handle.refetch();
handle.startPolling();
handle.stopPolling();

[stubailo]

@Urigo, @kamilkisiela I think we should fix this ASAP. I was just talking to @purban and it’s a bit weird because people from Angular 2 are going to be expecting RxJS, not a regular old observable. With this merged, at least we can implement a feature to replace the observable shim with Rx.

[kamilkisiela]

#430