Boy: add support for JDK built-in script engine

Cf issue #528
I did not remove the Mozilla Rhino implementation because I cannot be sure that using the JDK built-in script engine will not introduce regressions with existing scripts. Can someone check this with existing scripts ?
I introduced a hard-coded variable to select the javascript engine (Mozilla Rhino or JDK, default value is JDK). If there is no regression with existing scripts then I will completly remove Mozilla Rhino. If not, this engine selection variable could be converted into a preference.