Adding support for service factories in IServiceDescriptor

Fixes #64
I think making these setters private is absolutely the right thing to do™, but this will break other projects.

I just noticed these properties are being set in [OptionsServiceCollectionExtensions](https://github.com/aspnet/Options/blob/1.0.0-alpha4/src/Microsoft.Framework.OptionsModel/OptionsServiceCollectionExtensions.cs#L20).