Webview

@slluis - here is a simple WebView implemented for Mac and WPF. I can add other methods and events as needed, but I figured I'd go ahead and make a pull request and get feedback from you.

* For now this just lets you set the `Url` property and it navigates to that URL.
* There is no GTK backend for this.
* The default url in the sample is http://www.xamarin.com, and running this on Windows you'll see a popup error related to jQuery 1.9 or something. I don't know if there is anything we can do about that from WPF/Xwt.