data transfer: first implementation

First implementation of Reliable Data Transfer feature in Ring.

This implementation is a draft, comes with a Python script
tools/dringctrl/sendfile.py to play with and doesn't
implement all the API as described into "datatransfer: API proposal"
commit.
This version uses TLS over TCP-TURN sockets to encrypt data.

Transfers require a TURN server with TCP peer connections
as described by the RFC-6062.

Testing: Currently only sendFile API is implemented and data are
saved into a temporary file saved in "/tmp/ring_XXXXXX",
where XXXXXX are replace by mkstemp() command.

Change-Id: I5b8f48432edd58df5046e368a99f58ea44046dcd
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>