include example of observing and editing compilation phase during class loading

I've attached an example of observing the compilation process.
This came out of the discussion here: http://www.nabble.com/Groovy-as-arithmetic-expression-evaluator--to19459935.html

This example code is in /src/examples/groovyShell

The class is called ArithmeticShell. "The arithmetic shell is similar to a GroovyShell in that it can evaluate text as code and return a result. It is not a subclass of GroovyShell because it does not widen the contract of GroovyShell, instead it narrows it. Using one of these shells like a GroovyShell would result in many runtime errors." It should how to observe the compilation process and how to walk the AST.

I did not do a static import on java.lang.Math because it was a hassle to test. I did not extend CodeVisitorSupport for reasons stated in the Javadoc.

It includes a GroovyTestCase but does not include an Ant script to build it.

Perhaps this can help someone.