No longer use pbr for packaging (issue #12)
Drop argparse from install_requires

Don't add unnecessary external dependency on argparse, since it is in
the stdlib for Python 2.7+. For 2.6 it's an external download, but there
is no reason to default to using an external version while the stdlib
version is just fine.
Fix calling the program without a subcommand under Python 3

Under Python 2, the parse_args() call exits the program if no subcommand
was specified, but under Python 3 this call succeeds, resulting in a
missing '_func' key in the returned dict. Fix that by handling that case
ourselves.
Update README for 0.6
Bump version to 0.6