Configuration handler to go from default -> config files -> environmental variables -> command line options.

Adds an option to dump the current loaded config as a config file, and
the ability to expand environmental variables in help msgs.  Options
that come from a config file MUST be in a config group.

Extends/depends on os, ConfigParser, and optparse.

Advanced functionality of ConfigParser and optparse HAS NOT BEEN TESTED.

Usage example:

    from ConfigManager import ConfigManager, ConfigGroup

    appname = 'cfgmgr'   # could use basename, I suppose

    cfgmgr = ConfigManager()
    cfgmgr.set_config_files(['/etc/%s/%s.conf' % (appname, appname),
                             os.path.expanduser('~/.%s.cfg' % appname)]

    daemonGroup = ConfigGroup(cfgmgr, "Daemon", "Settings for running as a daemon.")
    daemonGroup.add_option('-p', '--pidfile', envvar="CFGMGR_PIDFILE",
                           help="The location to use for the pid file.",
                           default="/var/run/%s.pid" % (appname,))

    cfgmgr.add_option_group(daemonGroup)
    print cfgmgr.parse_args()