Concurrent request issues

Here's a PR of my original branch for comparison, apologies for not submitting it sooner.

I'm concious there are other occasions where we're potentially registering to the container at run time, but I'd need to dig into the Nancy code to see where these methods are called. This would just get us a bit closer to the goal of a more efficient StructureMapNancyBootstrapper.

This is not related to issue #19 as that seems to have been resolved by a recent StructureMap release, this is in relation to suggestions made by @jeremydmiller as to how the Bootstrapper can be improved.