If Sync is (likely) not configured, the only loaded JS is for the XPCOM
service itself.
The UI code is now smart enough to initialize the Sync service if it
isn't yet loaded. This addresses bug 825728.
Clients was being exported and used as a singleton. We still use Clients
as a singleton in some places, but only in test code. The preferred
method to access Clients is now through a service instance.
Weave.Clients is no longer exposed. Callers go through Weave.Service
now.