Adds support to avoid VRaptor stack if needed

When migrating a large project from another MVC framework to VRaptor in some cases maintain both frameworks configured in the same web.xml is needed during the migration period. In some cases It would be preferable to configure VRaptor filter with /* mapping and then programatically choose to trigger VRaptor stack or not. With VRaptor 3 this can be accomplished by extending VRaptor filter and calling chain.doFilter(...) when needed.

I am currently migrating a project from Struts 1.3.7 to VRaptor 4 and I've customized Struts to let CDI create It's Actions and sometimes HttpServletRequest dependant dependencies are injected in those Actions.

As a migration strategy I opted to use VRaptor 4 only to respond to requests with mobile user agent (to be able to accomplish a mobile first approach). VRaptor then must be the first to answer the request but It should trigger It's stack only for mobile requests. When a route doesn't match any VRaptor Controller in a mobile request it should be propagated to Struts (will take some time to migrate all Actions and as soon as one Action is migrated to VRaptor that change goes to production).

This pull request intent to facilitate programmatically avoid VRaptor stack if necessary by specializing the RequestStartedFactory and firing any event that differs VRaptorRequestStarted and implements RequestStarted.