Basic support for Tacx Vortex trainers

This adds support for controlling & receiving telemetry from Tacx Vortex trainers.
See the manufacturers page for more details: http://www.tacx.com/products/trainers/vortex-smart

This *should* work with other Vortex models than just the most recent Smart version, too, but I don't own these and can't test correct operation.

One caveat is that the Vortex brake only supports a power setpoint and can't itself take slope into consideration. So ideally we'll want to calculate the load for the current athlete at a given slope and send that to the brake, but that is not currently implemented.