config.py: allow specifying repo uuid in config

Now repository sections have optional 'uuid' parameter to specify repo uuid.
server.py: report exception stacktraces to client

To ease debugging and error reporting, dumb exception stacktrace to users
server.py: suppress exception when client is lost

Workaround bug http://bugs.python.org/issue14574 by suppressing exception from StreamRequestHandler.finish
WIP: backend rewrite