Display help more liberally.

Plow now displays help if you:

 * don't enter any args or commands
 * enter "help" or "usage"

It will continue to display help if you do:

    plow --help
exit with 0 on help
use return code EX_USAGE (64) from sysexits.h

Similar to other commands such as mkdir
Update exit status to follow bash conventions.

Previously, it explicitly returned 0. As this is the default for exit it
confusing and unconventional.

Also added a note explaining the meaning and source of exit 64.
Merge pull request #7 from MarkBennett/add-help-on-usage

Display help more liberally.