Support libusb_set_option in additon to libusb_set_debug. (#39)

* Support libusb_set_option in additon to libusb_set_debug.
The latter is deprecated in libusb 1.0.22 and later.

* Move the "debug level constants" todo to the right place. Add a comment
about libusb 1.0.22 exported API version.

usb.go

@@ -134,6 +134,8 @@ type Context struct {
 
 // Debug changes the debug level. Level 0 means no debug, higher levels
 // will print out more debugging information.
+// TODO(sebek): in the next major release, replace int levels with
+// Go-typed constants.
 func (c *Context) Debug(level int) {
 	c.libusb.setDebug(c.ctx, level)
 }