[REFACTOR] deleted IncomingTransferObject interface

Renamed the BinaryChannelTransferObject to BinaryXMPPExtension so it
should be easy enough to figure out what is send over the wire.

Change-Id: I9038e4583279b496cd7eca9cd239461532e06618
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1509
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>