Added some initial docs & made the CLI tool work.

This commit (which depends on #5) adds some initial documentation via the Sphinx (http://sphinx-doc.org/) tool, a standard doc tool in the Python community. There's not a lot there, but there's at least autodocs for all the existing modules & the start of a tutorial.

I'd recommend setting up a Read the Docs account (https://readthedocs.org/), which does automatic Sphinx doc hosting. You setup a post-commit hook on your repo & on every push, your latest docs will get automatically built/hosted for you.

I also had to adjust the name of the CLI tool. With the revised package layout, I was hitting import collisions (because everything was named ``s2protocol`` & imports were confused).