Added test request file for nc.
Added debug output for usb::transfer constructor.
Implemented completely asynchronous send_async.

+ Now using new usb::transfer for every cp210x::send_async.
+ transfer_completed handler in cp210x::send_async
  explicitly calls reset on shared_ptr<usb::transfer>
  to force last remaining reference to transfer disappear
  immediately.
Changed akemi.cpp according to changes in cp210x.

+ Akemi project uses manual reading mode of cp210x and
  server as a test of its correct work.
First implementation of asynchronous usb-tcp adapter.

+ Uses boost::asio and libakemi_usb.
+ Can handle multiple clients at once for the same usb connection.