Automate java version selection depending on the version of the restlet framework

Forked from #874 : java 6 for 2.2, java 7 for 2.3 and above

workload: 1h analysis + 3h implementation

Assigned to JLO for roadmap priorization

[jlouvel]

Let's do it for 2.2.1 and 2.3 M2. Assigned to @thboileau

[thboileau]

note: setting the JAVA_HOME environment property should be enough.