Find and document rules concerning "extra" requires and "test_require"

Quoting Jim:

I'd like us to say something along the lines of:

"If your tests have additional dependencies, define "test" extra with these dependencies. Also define tests_require and test_suite so that your package supports the test command."

(Not verbatim and we should give examples.)

Note I used to object to tests_require because the information wasn't available after installation. I still do, but I've started using it anyway. Tres wrote a plugin to fix this (thanks Tres), but I think it's easier just to define tests_require (and test suite) along with tests_require.

Letting people run tests via the setup command is a lot better than trying to tell them how to use a buildout.