Scripting: Add script engine for lucene expressions

Adds support for new "expression" script engine.

This is backed by lucene expressions.  These are javascript expressions, which can only access numeric fielddata, parameters, and _score.  They can only be used for searches (not document updates).

closes #6818
Ok I made the dep optional.  I didn't need to change anything else since the lib starts with "lucene" and lucene* is already included.  I checked the tar.gz and it includes all 3 deps.