Include referenced assemblies in search for resources

This should resolve the issue mentioned in https://github.com/fsprojects/FSharp.Data.Toolbox/pull/4

The problem is that `Assembly.Load("FSharp.Data.Toolbox.Twitter)` fails. I'm not entirely sure when this should work and when not (i.e. when is the assembly already loaded and .NET can find it and when not).

However, the assembly is also listed in the `ReferencedAssemblies` and so this change looks for the resources there too (I'd think that the assembly will always be in referenced assemblies, because you need to reference the assembly you want to use! ...but I'm not sure I fully believe this...)