Added Manifest for Listener

Added Manifest and Tests for Listener Element in server.xml. Updated the README and added an example for Tomcat 7 Instance with JMX Listener.
I'm wondering if this can maybe be done more like https://github.com/puppetlabs/puppetlabs-tomcat/blob/master/manifests/config/server/valve.pp#L22-L23 to allow greater flexibility for nesting the listeners?