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)
Can the title please contain "FSharp.Core"?  I think "FSharp.Core for F#" is ok. It would be good if the description clarified which version to use for F# 3.1+ and which version to use for F# 3.0+.  

Note also the use of "+" - you can use the F# 3.0 runtimes (4.3.0.0 etc.) when compiling with the F# 3.1 compiler.  Indeed that's pretty much recommended when building a library that you want usable as broadly as possible.