Fix TAG_NAME interpolation

Add support for Go modules (google#77)

This is nice for anyone who wants to depend on this library, but
also required in order to make a Homebrew package for it.

Improved OAuth2l Client Interface (google#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.