Update to .NET API Client Library version 1.8.1

This update adds support for the latest Google API client libraries which just went to GA!

Note:
* You now will need the NuGet package manager to build the project.
* You can install the packages as described in Photohunt here:
https://developers.google.com/+/photohunt/csharp#step_2_set_up_the_photohunt_app