Adapt to API breaking changes in pyusb

pyusb.usb.util.getstring() does not accept a `length` argument anymore.
pyusb.usb.core.Device.read() and write() methods do not accept an `interface`
argument anymore.

This is an issue/pull request. I went with the changes that broke my usage of pyftdi. I haven't groked everything about pyftdi or pyusb, so it may not be exhaustive or correct. It also obviously break pyftdi for pyusb <= 1.0.0b1.