add AppVeyor 2 build

Adds AppVeyor 2 build support for Frank. In addition, upgraded to VS 2013, but should still work in VS 2012. It is still set to .NET 4 and F# 3.0. Removed the msbuild nuget stuff in favor of the nuget restore.

https://ci-beta.appveyor.com/project/ctaggart/frank
Copy the nuget feed link, if you want to try out the build.

![image](https://f.cloud.github.com/assets/80104/2305689/8afd157a-a265-11e3-858f-f59bf0e084d7.png)

![image](https://f.cloud.github.com/assets/80104/2305692/cd47a7b0-a265-11e3-8129-806e5df30f5f.png)