TurnTransport: better IO api and fixes

* API additions:
  - peerAddresses
  - readlinefrom
  - writelineto
* API changes:
  - recvfrom: char*/length version
  - sendto: per-peer call, not longer a map
* Add more API documentation
* Max TURN buffer size changed to 4096 bytes
* Better IO buffer management with peers
* Fix auth data (was temporary buffer pushing garbage values to server)
* Turn tests modified for API changes

Change-Id: I0bffe114301e8cb1e2f2e37d7a0eb5ba67f38c61
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>