Simplified INancyEngine to a single HandleRequest method. The previous 'overloads' are now
provided by extension methods. Various fake INancyEngines in the tests were correspondingly updated.
Add CancellationToken paramater to INancyEngine.HandleRequest(). NancyOwinHost invokes with owin.CallCancelled enviroment item. Updated fakes accordingly.
Don't store the request cancellation token in the NancyContext.Items - not used nor required.
Merge pull request #1386 from damianh/OwinCallCancelled2

Support for request cancellation in INancyEngine and owin.CallCancelled (redo)