Dont access nashorn classes and Engine scope
Lazy wrapping of the variables in the script bindings for nashorn
Select the engine according to the JDK

When the setting "script.javascript.engine" is not specified or equal to
"auto", the engine selected depends on the Java version: Nashorn for Java
8 and Rhino for Java 6 and 7.

Also take extra care to not load the nashorn or rhino classes
at all if the engine is not selected.
Trivial cleanup