Fix for Debug startup project for VS.

As mentioned by @waylaidwanderer in #252. The startup project defaults to protobuf-net on a fresh clone (because of link below). This fixes that for visual studio. I'm not sure if it has any effect on MonoDevelop.

See [this post] (http://stackoverflow.com/questions/694730/why-is-set-as-startup-option-stored-in-the-suo-file-and-not-the-sln-file)