get_url returns a fully-qualified URL
'host' returns http://www.habariproject.org
'habari' returns http://www.habariproject.org/habari, if you
have Habari installed into a /habari/ sub-directory
'site' returns http://www.habariproject.org/site if
you are installing with a subdirectory path
'user' returns one of the following:
http://www.habariproject.org/user
http://www.habariproject.org/user/sites/x.y.z
'theme' returns one of the following:
http://www.habariproject.org/user/themes/theme_name
http://www.habariproject.org/user/sites/x.y.z/themes/theme_name
'admin' returns http://www.habariproject.org/admin
'admin_theme' returns http://www.habariproject.org/system/admin
'login' returns http://www.habariproject.org/auth/login
'logout' returns http://www.habariproject.org/auth/logout
'system' returns http://www.habariproject.org/system
'vendor' returns http://www.habariproject.org/system/vendor
'scripts' returns http://www.habariproject.org/system/vendor
'3rdparty' returns http://www.habariproject.org/3rdparty
if /3rdparty does not exists, /system/vendor will be returned
'hostname' returns www.habariproject.org