Added Rackspace Cloud Files support

To use Rackspace Cloud Files, run setup and provided the following
details when asked:

username: <your Rackspace username>
password: <your Rackspace password> # NOT THE API KEY

For US accounts, the auth enpoint is:

auth_url: https://identity.api.rackspacecloud.com/v2.0/tokens

For UK accounts:

auth_url: https://lon.identity.api.rackspacecloud.com/v2.0/tokens