Targets dotNet 4.0

I targeted this project to dotNet 4.0 with the idea that perhaps another Nuget package could be created that would work against this framework.

Let me know if there are any changes you'd like to see before accepting this pull request. I realize that it would be better to merge this into a parallel branch from the master branch, but my github skills are limited so far. In any case, I'm happy to do what's needed if you think this adds any value.