Added listX() methods to replace uses of lists()

This pull request addresses #69.

The method name lists() is highly unconventional and unintuitive, so I set out to change it on my way to adding some submodule features.  I have maintained compatibility with code that already uses the lists() methods.

I have not added new unit tests for XCommand classes that did not already have them.  My reasoning was that I am not really adding new functionality here, so the need for additional coverage is very low.