Use email address instead of session id to identify users

Fixes issue #220 by refactoring the AppLoadBalancer to use e-mail addresses when creating and getting tokens instead of the Rails session ID. The problem was that, in certain deployments (including VirtualBox), the session ID was nil, causing a SOAPFaultError when contacting the UserAppServer (which was expecting a string and not nil).

Tested on a one node deployment on VirtualBox, and on EC2.