Argparse and dynamic command loading support

This pull request converts from optparse to argparse for improved argument handling and support for command-specific arguments.  ConfigParser support was also integrated with argparse so config file values influence command-line defaults.  

In addition, commands are dynamically loaded and a modified BaseCommand object should make it easier to add new commands.  

This pull-request introduces the following commands:
 * me
 * organizations (list, create, update, delete)