Parallelize tests (#17)

* Store a reference to libusb implementation in the context, transfers
and some more places. Remove the global libusb variable.

* Parallelize tests.

* Fix the link in README.

endpoint_stream.go

@@ -17,7 +17,7 @@ package gousb
 func (e *endpoint) newStream(size, count int, submit bool) (*stream, error) {
 	var ts []transferIntf
 	for i := 0; i < count; i++ {
-		t, err := newUSBTransfer(e.h, &e.Desc, size, e.Timeout)
+		t, err := newUSBTransfer(e.ctx, e.h, &e.Desc, size, e.Timeout)
 		if err != nil {
 			for _, t := range ts {
 				t.free()