OPENJPA-2409 enable persistenceXmlFile parameter to handle classpath resources as well.

contributed by rmannibucau, txs!
applied with small changes and doc improvements


git-svn-id: https://svn.apache.org/repos/asf/openjpa/branches/2.3.x@1533462 13f79535-47bb-0310-9956-ffa450edef68