GROOVY-4877: Groovy Console should set the context class loader to the class loader that loaded the script.

Incorporated Graeme and Pauls feedback. The classloader is now only modified for the scope of execution of the script and only if the "useScriptClassLoaderForScriptExecution" is true. "useScriptClassLoaderForScriptExecution" defaults to false and is set to true in the main method.