Add support for scripting in Scala

Some people would like scala support in XWiki...

At this point in time it seems there's no JSR223 support for Scala except in the Sling project.
Here are pointers I've found:

• https://issues.scala-lang.org/browse/SI-874
• http://stackoverflow.com/questions/5654888/how-do-i-set-up-jsr223-scripting-with-scala-as-scripting-language