Add integration tests for Tomcat

As a follow up, we could easily adapt this test suite to
run tests towards a WAR run by jetty runner.
Remove old, simpler integration tests
Rewrite tests to have both tomcat and jetty
README.md: update test instructions
README.md: allow to copy and paste instructions
web.xml: add listener entry for Jetty

Apparently Jetty is not able to honor the @WebListener annotation
so we need to add a <listener> field to make it work.

Tested both using Jetty and Tomcat 8.
Add more stuff to .gitignore
Merge pull request #8 from bassosimone/feature/integration-tests

Implement integration tests