Exception on Windows Universal

On windows universal there is no WebClient avaliable. It throws an exception inside SetHtml All referencesz should be converted to WebRequest.

```
Exception thrown: 'System.TypeLoadException' in HtmlRenderer.dll
AppError:System.TypeLoadException: Could not load type 'System.Net.WebClient' from assembly 'System, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e, Retargetable=Yes'.
at TheArtOfDev.HtmlRenderer.Core.Handlers.ImageDownloader..ctor()
at TheArtOfDev.HtmlRenderer.Core.HtmlContainerInt.SetHtml(String htmlSource, CssData baseCssData)
```

Start at the ImageDownloader constructor and HtmlContainerInt.SetHtml entry point, then inspect where WebClient is referenced in the Windows Universal path. Reproduce SetHtml on Windows Universal and verify that it no longer throws the reported TypeLoadException and that the relevant references use WebRequest.

Vom Indexierungsmodell aus dem Issue-Text verfasst.