CAY-1883 - Clean up Cayenne maven structure - get rid of aggregate modules

* cleanup, killing unused aggregator plugin, being explicit about the scope of
  various dependencies
* jdbc4-compat no longer needed under Java 6

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1542588 13f79535-47bb-0310-9956-ffa450edef68