migrate FSharp.Core.Microsoft.Signed to FSharp.Core

fix fsharp/fsharp/#352

These `nupkg`s will be identical, except their `id`s are `FSharp.Core`.
FWIW I don't think we need both the "3.0.0.0" and "3.0.0.1" versions, nor both "3.1.1.0" and "3.1.1.1". 

Can you just start with a clean slate and use "3.0.0.0" (containing the contents listed in the latest FSharp.Core.Microsoft.Signed.3.0.nuspec) and "3.1.0.0" (containing the latest contents listed in FSharp.Core.Microsoft.Signed.nuspec)?