Better way to use StructureMap to get nancy module

Do  not mutate the container's registrations while resolving nancy modules (ie. running the web app)

Code [as per gregsochanik's branch]( https://github.com/gregsochanik/Nancy.Bootstrappers.StructureMap/blob/concurrent-request-issues/src/Nancy.Bootstrappers.StructureMap/StructureMapNancyBootstrapper.cs#L188)  and jeremydmiller
as discussed here:  https://github.com/NancyFx/Nancy.Bootstrappers.StructureMap/pull/20#issuecomment-49009613