streaming core part 1

we have the stream callback contract
and a Tee that can stream from one source to multiple destinations
First hook on tox backend

It works, still have very rough edges.
implemented outgoing transfers

With a reference implementation with the tox backend.
Better doc and fixed weird param on botplugin.
make pep8 happy again
try to make 3to2 understand pipe
python2 bufferio compatibility
Merge pull request #295 from gbin/streams

Implemented a file transfer API
Please give it a try !