adding support for AppVeyor & Source Indexing

This adds support for building on AppVeyor 2 beta and source indexing using my SourceLink project. See my blog [Source Indexing FSharp.Compiler.Service on AppVeyor](http://blog.ctaggart.com/2014/02/source-indexing-fsharpcompilerservice.html).

If you look here, you will see that both Travis CI and AppVeyor are working.
https://github.com/ctaggart/FSharp.Compiler.Service

You can try out the source indexing by adding the NuGet feed listed on the AppVeyor page:
http://ci-beta.appveyor.com/project/ctaggart/fsharp-compiler-service