Update s3 customizations for help docs

This PR updates the s3 customizations to show the examples.  There is a bit of duplication here which I believe can eventually be cleaned up.

In the mean time I've started a test suite that checks command/subcommands for example docs in the help output.  This will at least catch regressions in the future.

In order to add this test suite I've moved the base classes for help outputs into a tests.__init__ package, as this is where the other test base class lives.

cc @garnaat