servo-lab accepts a "global" range in args.

eg.

Setup servos for 9, 10,  11, 12. Apply a limited range of 45-135degrees

node utils/servo-lab.js 9 10 11 12 R:45:135

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>
Drop upValue and downValue from Button

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>
Servo "inversion" support

When dealing with multiple servos eg. wheels, legs etc. servo positions may be opposing ie. `0` on one servo is `180` on its opposing servo. To make it easier to reason about, the Servo class now accepts an optional `isInverted` property which will automatically invert servo position values before writing the value to the device.

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>
v0.7.8

- Servo position value inversion
- Removal of (up|down)Value from Button

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>