Replace Jetty-specific Request with HttpServlet{Request,Response} in InstrumentedHandler

Some frameworks like [Atmosphere](http://async-io.org/) bring their own `ServletRequest` and `ServletResponse` implementations which clash with Jetty's `Request` class.

This PR replaces the Jetty specific `Request` class with the standard `HttpServletRequest` and `HttpServletResponse` interfaces. This should fix #444 and similar bugs.