[SystemUpdate]: handle Openstore app updates
Created by: mardy
Change the client API URL to point at the OpenStore, and update the JSON parsing to match the Openstore's format. Remove all the code related to the SSO and token download, and remove the now unneeded dependency on libUbuntuOneAuth. We can bring it back later, should we ever find the need.
The Openstore support can be improved once the two following bugs are fixed: