argparse is the successor to optparse, and has a lot of nice additions

we may want to take advantage of. It knows how to do things like

optional values to arguments flags, variable argument values, and

"positional arguments" (such as a command name), reducing the amount of

work we have to do.
The two are pretty compatible, allowing for most of the porting effort

to remain under the hood. New commands can certainly take advantage of

the new support though.
We still maintain our own Option and OptionGroup wrappers, which we

still need in order to map to configuration keys.
argparse is natively available on Python 2.7 or higher. For older

versions, we depend on the 'argparse' package from PyPI, which provides

compatibility with Python 2.7's argparse.

Tested with both the native argparse in 2.7 and the downloaded version

in 2.6 (fetched from running python2.6 setup.py develop).

Note that argparse claims compatibility with 2.3 on, so we should be in good shape.
Checked the help output for every command.
Tested using all variations of --help (for both global help and per-command).
Tested calling commands with flags and positional arguments after the command

name.
Tested calling commands with attempting to use flags before the command name,

which correctly resulted in an error.
Tested with custom scripts in the patch (using rbt-mycommand names). They got

the correct arguments.
Checked with the new guess support. Instead of needing to do the hacky argv

manipulation from the first change, we can now nargs to say it optionally takes

an argument, simplifying the code.