Command "clear" for the text InteractiveShell

When we use groovy as a scripting tool in production, we often use
the InteractiveShell to do some basic tests to prove some concepts,
so we often try different ways and sometimes we forget what we have
typed.

Of course, we can use command 'display' and check what have typed,
but often it is more desirable to simply clear all typed statements
and restart!

For that purpose, I have patched the class "groovy.ui.InteractiveShell"
to add another command 'clear'. I have used the modified version for some
time and now I upload this patch.

This patch is created again the latest version code (InteractiveShell $Revision 1.25)