Request startup

Added IRequestStartup to serve the same purpose as IApplicationStartup but for running at the beginning of each request. In addition to the pipelines it also gets passed in the current NancyContext. The IRequestStartup tasks are executed before RequestStartup so the user bootstrapper retains the last say on things.

For request container based bootstrappers the IRequestStartup instances are resolved from the request container.

Not sure I see the point of this, but people have asked for it so there you go :shipit: