Convert USB Speed string to an enum

[mbanth]

In a comment in Pull Request #103, @xross suggests representing the usb_speed in helpers.py as an enum. Because the tests currently use Python 2.7 which does not include enums as part of the language, we have decided to defer this enhancement.

Python 3.4 and later versions support enums. Once the tests have made the transition from Python 2.7 to Python 3.4 or later (almost certainly 3.7 or later), we can convert usb_speed from a string to an enum.

Issue #83 tracks the transition from Python 2 to Python 3.