added F# Make build.fsx

Added an F# Make build.fsx. It provides source indexing and the ability to run the build from the command line. AppVeyor continuous integration support is included. I also added source indexing using my SourceLink project. This allows you to step into the source in a debugger like Visual Studio. It will download the source code from GitHub on demand. 

AppVeyor makes the NuGet [artifacts](https://ci.appveyor.com/project/ctaggart/autofaccontrib-nsubstitute/build/3.3.3-ci1406200443/artifacts) available on this feed: `https://ci.appveyor.com/nuget/autofaccontrib-nsubstitute-xjex2s5r35lf`. Feel free to try out the build.

```
PS C:\Projects\AutofacContrib.NSubstitute> .nuget\nuget.exe install AutofacContrib.NSubstitute -OutputDirectory packages
PS C:\Projects\AutofacContrib.NSubstitute> .nuget\nuget.exe install AutofacContrib.NSubstitute -OutputDirectory packages -prerelease -source https://ci.appveyor.com/nuget/autofaccontrib-nsubstitute-xjex2s5r35lf
```

![image](https://cloud.githubusercontent.com/assets/80104/3336698/a29ee0f4-f838-11e3-9caa-f66477845b1f.png)

I'm sure you will have questions. Here to help.