DEFAULT_CONFIG_FILE => CONFIG_FILE.

This refactor makes sure that there is only one config file. Therefore,
"Default" no longer made sense.
Update the credential loading to be fog-style.
Update the client to use the proper credentials.
Add a configuration section to the README.
Update the help and error messages.
Major version bump v2.0.0.
Merge pull request #2 from swipely/add-fog-style-credential-loading

Add fog style credential loading