FileWatcher sample

[LeeCampbell]

Provide help to questions like [http://codereview.stackexchange.com/questions/89961/rx-net-file-watcher]

Walk through the issues with FileSystemWatcher and then link back to polling solution from #22

[LeeCampbell]

var fsw = new FileSystemWatcher(".");

var fileChangedStream = Observable.FromEventPattern<FileSystemEventHandler, FileSystemEventArgs>(
    handler => fsw.Changed += handler,
    handler => fsw.Changed -= handler)

[LeeCampbell]

https://aspnetcore.slack.com/archives/rx/p1481549187000056