add a data() method, to avoid reaching into any xfer fields.
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.
complain about the incompatible pointer types.
Use pkg-config to add correct include paths for libusb regardless of the platform.