Add operator overloading section to Groovy specification.

The purpose of the spec is also to be executable, so if we speak about the various method / symbol mappings, we need to ensure that that mapping exists. That's why it'd be ideal if we could indeed have a (even simple) test for each.
You can include a line of a script in the table just like you include other scripts. But I'm not sure how we could really just restrict to the a.plus(b) and a+b expressions for inclusion. Hmm...