Changed Rhino implementation to evaluate scripts in a context instead of...

... the Rhino shell (which scope is mutated on each eval call!). RhinoShell has been renamed to RhinoExecutor as a result. Also fixed exception catching with Rhino engine (there was no test for this) and improved RhinoSpec to test for CommonJS support.

Fixes #20 and #21.

(I signed the Typesafe CLA)