Fix for Debug startup project for VS.

See http://stackoverflow.com/questions/694730/why-is-set-as-startup-option-stored-in-the-suo-file-and-not-the-sln-file.

This is for Visual Studio only; it's unknown if this will work for MonoDevelop.
Merge pull request #260 from cwhelchel/fix/startproj

Fix for Debug startup project for VS.