Added switch -base to export_vars, which will make export_vars return a complete URL. The advantage of using this is that it'll be smart about only appending the question mark if there were any variables to export.
Added -no_empty switch to export_vars. If specified, variables with an empty string value will be suppressed from being exported. This avoids cluttering up the URLs with lots of unnecessary variables. If we weren't this close to an important release, I would advocate making -no_empty the default behavior.