Async support with FromAsync not ToObservable

[LeeCampbell]

As pointed out in this post:

http://stackoverflow.com/questions/18477018/rx-and-tasks-cancel-running-task-when-new-task-is-spawned/18493158#18493158

ToObservable does not really offer Cancellation support as there is no way to pass in a CancelationToken. Favor the Observable.FromAsync(Func<CancelationToken, Task>) method instead.