Adding Upgrade guide

I've gone through all the [acceptance tests](https://github.com/rackspace/gophercloud/tree/release/v0.1.1/acceptance) in our `releases/v0.1.1` and added new code for all the old features that we supported. Hopefully this should be enough.

Feedback welcome :smile: 
"`gophercloud.ServiceClient` client" sounds redundant. Maybe just "retrieve a `gophercloud.ServiceClient`."?