Allow configuring the NancyOwinHost to support pasthrough if the specified status code(s) occur and test.

Notes:
- Add INancyEngine HandleRequestAsync that returns a Task<NancyRequest>. The other HandleRequest
method is marked obsolete.
- NancyEngine HandleRequestAsync uses TaskFactory instead of ThreadPool.QueueUserWorkItem. The
obsolete HandleRequest invokes HandleRequestAsync and calles onError/onComplete delegates in the
continuation.
- Added seperate Nancy.Owin.Tests project, didn't want modify Nancy.Hosting.Owin.Tests. The
NancyOwinHostFixture here uses Microsoft.Owin.Testing and Owin.Extensions for a super simple test. No
mucking around with environment dictionary required. While Microsoft.Owin.* packages have the non-windows
restrictive distribution, but they are only used in tests so isn't a problem.