[imp #2] Update SynchroServer interface

Change session() function return type: instead of plain String, we now expect a
Try[String]. This allows error processing and handling at caller level.
[imp #2] Add synchronization API

New synchronization protocol uses "/papers/$paperId/sync"
Mobrite-compatible protocol uses "/papers/$paperId/q"
Merge remote-tracking branch 'upstream/master' into wip-web-client