Improved OAuth2l Client Interface (#70)

-Uses advanced go-flags library to parse command-line flags, which
improves organization, flexibility, and better built-in help doc.
-Added explicit flags for GUAC arguments such as scope and audience.
-Added support for a new "curl" command.
-Fixed variadic bug in sso.go that incorrectly handles multiple scopes.

Update for v0.9.0 release.