The preferred way to install this extension is through composer.
Either run
php composer.phar require --prefer-dist promocat/rackspace "*"
or add
"promocat/rackspace": "*"
to the require section of your composer.json file.
new Rackspace(Rackspace::US_IDENTITY_ENDPOINT, [
'username' => {username},
'apiKey' => {apiKey},
]);