Create an async/IObservable Ping function

[govert]

Motivated by a recent Stackoverflow discussion, I thought this would be a nice sample function to discuss the async / IObservable features.

[govert]

https://github.com/RxDave/Rxx/blob/master/Main/Source/Rxx.Bindings-Net451/System/Net/NetworkInformation/ObservablePing.cs