Owin WebSocket adapters

#96
Adapters for OWIN->AspNet and AspNet->OWIN WebSockets. Note this can only be done via middleware, but it also needs to integrate with the environment/context adapters.  Tested with Nowin.
We should also offer an extension method on AddMiddleware (that's the new pattern for the Owin middleware). Using this in our stack would look like:

```C#
public void Configure(IBuilder app)
{
    app.UseOwin(owin => 
    {
        // Put the web socket adapter in the pipeline
        owin.UseWebSocketAdapter();
    });

    app.Run(async context =>
    {
        if (context.IsWebSocketRequest)
        {
            var webSocket = context.AcceptWebSocketAsync();
        }
    });
}
```