FSharp.Core 3.1.2.1

This fixes #358 and fixes #359. I upgraded the 3 assemblies that were still 3.1.1 to 3.1.2. I figured out how to get the FSharp.Core assemblies built for MonoAndroid and MonoTouch on Windows. I added them to the NuGet package. I made it so AppVeyor builds the NuGet package.

![image](https://cloud.githubusercontent.com/assets/80104/5333567/36e8f560-7e30-11e4-9057-48ec8eeda848.png)