Fix #3: Added Sphinx documentation support

This commit includes updated docstrings for both cmdparser.py and
datetimeparse.py and also Sphinx sources and makefile to generate HTML
and other documentation. I've also included a custom theme "graphite"
which is a light-on-dark theme that I find reduces eyestrain.

As well as documentation changes, this commit also includes a new unit
test as "named tokens" weren't being tested previously.