Perform logout API call on logout

Unfortunately this doesn't work because we fire off the API call
in a service and then perform the HTTP request in an AsyncTask. By
the time we are going to include the user's authentication token
the user has been logged out of the app so we don't have the token
anymore.
Merge branch 'maven' into logout-api-call
Fix logout API call

By explicitly providing the auth token to use we don't need to check
the currently logged in user.
Merge branch 'logout-api-call' into maven
Change references to session to auth token
Set the user agent for all API calls

This gives us the ability to track API calls by different versions
of the app.