Switch from DuplexStream to raw Senders and Receivers

`DuplexStream` is now deprecated.

Closes #67.
Merge pull request #68 from bjz/dewarningify

Switch from DuplexStream to raw Senders and Receivers
swap_ack implemented with controllable queue size
Added Travis icon to README
Merge pull request #69 from kvark/ack

swap_ack implemented with controllable queue size
Hacky legacy OGL compat