ContextListener class for OpenEJB startup in web container environments

Create ContextListener class for easier integration with web containers. When a web application has started up, the web container notifies listener-class'es so they can do some after-web-startup-before-first-client tasks like initializing openejb.

Unless I'm mistaken, OpenEJB supports Tomcat only with a hack so openejb libraries are added to an appropriate classloader. The listener-class could easy it greatly.

<listener>
<listener-class>org.apache.openejb.webapp.OpenEJBContextListener</listener-class>
</listener>