Move to Java 7 as minimum version

Creating the issue to link with other issue and list pros/cons.

Pros

Java 6 is end of life since February 2013.

Required for:

• Jetty 9, see XWIKI-8904
• Tomcat 8, see http://tomcat.apache.org/whichversion.html
• JGroups 3.5 (still a proposal to be confirmed, http://sourceforge.net/mailarchive/message.php?msg_id=31263813)
• Java 6 has bad support of Javascript JSR223 API, see XWIKI-5586
• Apache commons-lang 3 has started deprecate APIs that exist in Java 7 (for example ObjectUtils#equals)

Improved performances in various scripting languages thanks to new Invoke dynamic

Nice to have

• Strings in switch Statements. Interesting for various parsers.
• The try-with-resources Statement. Nicer code when playing with Closable resources.
• "Multi-catch". Allow catching several exceptions in the same catch block.

Cons

Still no used much in production.