persistence.xml: updated to be JPA 2.0

Based on http://en.wikibooks.org/wiki/Java_Persistence/Runtime#Example_JPA_2.0_persistence.xml_file

No functional changes.
pom.xml: don't run cobertura and jetty plugins when all tests skipped.

Fixed build under Travis CI where `mvn clean install -DskiptTests=true` command
executed before each build.