Upgrade to Servlet 3.1, Tomcat 8 and Jetty 9

Upgrade to latest versions of Tomcat and Jetty and to the latest Servlet
API whilst will remaining compatible with Tomcat 7 and Jetty 8.

Fixes gh-1832, gh-369
Add Tomcat 7 and Jetty 8 samples

Switch the Tomcat 8 and Jetty 9 samples to Tomcat 7 and Jetty 8. Since
Tomcat 8 and Jetty 9 are now the default versions the previous samples
are no longer relevant, however, we do need integration tests to check
that we remain compatible with Tomcat 7 and Jetty 8.

See gh-1832