Add wayback-sample-overlay

This is a new sub-project for OpenWayback and provides an example of a Maven 'overlay' over openwayback-webapp.

It also includes the following:
* working example and documentation on how to run with CDX server
* a sample warc and cdx file
* mvn configuration to be runnable with mvn jetty:run-war and mvn tomcat7:run-war

The README includes additional documentation info on the config.