add a data() method, to avoid reaching into any xfer fields.
Remove context close race condition
to sync on c.done channel before exiting.
Endpoint tests
check.
Transfer tests
newUSBTransfer.
instructions for windows are simpler now that we use pkgconfig.
detach before claim
Bump the Ubuntu and Go versions for Travis
descriptor in the transfer struct.
Use async transfer API everywhere
Don't assume libusb headers are always in libusb-1.0.