Fix issue #129 - fromEventHandler

[danien]

Explicitly specify 'EventArgs type parameters for Observable.fromEventHandler to ensure it returns IObservable<'EventArgs> instead of IObservable

[danien]

I'm not sure why the CI command-line build is failing with the following error:

System.IO.FileLoadException: Could not load file or assembly 'System.Threading, Version=4.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The located assembly's manifest definition does not match the assembly 
reference. (Exception from HRESULT: 0x80131040)
   at System.Console.get_OutputEncoding()
   at Program.main(String[] args) in D:\a\1\s\src\app\Fake.netcore\Program.fs:line 306

It happens locally as well if I try to build using fake.cmd on Windows and fake.sh on macOS Mojave, but I can build both FSharp.Control.Reactive and FSharp.Control.Reactive.Testing projects just fine and all tests pass in Visual Studio 16.4.3.

[panesofglass]

Thanks! I have to get the build fixed to publish, but will try to do so soon.