Add ability to retrieve HTTP status code from HttpResponse

The use case here is to be able to get the response status code
in a backend-agnostic way, i.e. without resorting to downcasts of
the underlying response object. This is useful when writing very
generic intents, e.g. for doing metrics on the request/response
cycle.
Bump servlet-api to 3.0.1
Use HttpServletResponse#getStatus to get status code for Jetty backend
Merge pull request #259 from BardurArantsson/add-getstatus

Add ability to retrieve HTTP status code from HttpResponse