Add to startup docs, remove argparse (no need for Python 2.7+)

`argparse` was added to Python's stdlib in 2.7.x, and doesn't need to be listed as a dependency. It actually messes up typical installations because pip will whine that packages from external sources aren't allowed by default. 

If you really wanted to, you could use `--allow-external argparse` to let pip draw from googlecode (where the external argparse package is being hosted instead of PyPI) but I say Python 2.7.x+ is a reasonable expectation :smiley:  

Also adds to start up documentation.