Compile libsass for one platform at a time

Added an inline MSBuild task, which make use of a new CoreFX API
`System.Runtime.InteropServices.RuntimeInformation` to obtain OS
description and architecture information, then translates into
`LibSassPlatform` (Win32 or Win64 etc.).

Also added ability to cross compile, e.g. Win32 on 64-bit system.

Additional OSes will be added with .NET Core support.

Added cross compile configurations for AppVeyor CI.

Author: am11

LibSass.NET.Tests/LibSass.NET.Tests.csproj

@@ -117,7 +117,7 @@
   </Target>
   <Target Name="AfterBuild">
     <Copy
-      SourceFiles="$(LibSassPath)libsass32.dll;$(LibSassPath)libsass64.dll"
+      SourceFiles="$(LibSassPath)libsass.dll"
       DestinationFolder="$(OutputPath)"
       SkipUnchangedFiles="true" />
   </Target>