Fix grabbing token

I came across this while looking for projects that assume the case of `X-Auth-Token`.  There's actually two issues:
- HTTP header names are case insensitive according to [rfc 1945](https://tools.ietf.org/html/rfc1945#section-4.2)
- It was actually matching this: `vary: Accept, Accept-Encoding, X-Auth-Token, X-Auth-Key, X-Storage-User, X-Storage-Pass, X-Auth-User`

This is also using the Rackspace Identity v1.0 service.  It won't work with with the v1.1 or v2.0 services.  If it's possible, you should probably use one of the [supported SDKs](http://developer.rackspace.com/#home-sdks) or the [official clients](https://wiki.openstack.org/wiki/OpenStackClients).