Add operator overloading section to Groovy specification.

I'm not convinced about the need for examples with tests. There is already an example demonstrating how to implement a method to get operator support (with test). The table includes the other mappings.

Now, if it's possible to incorporate a line from a source file into a cell of a table, then the table can be validated. But if that's not the case, I don't see the value of lots of repetitive examples. Is it possible to include a line from source code in a table cell?