Add brightness control

The arduino library for neo pixels is quite nice in that it has a brightness control. This is a simple scaling that is applied to each pixel to control the overall luminosity of the pixel. This is an implementation of brightness for the raspberry pi library.

The leds array for the channel is unaffected by these changes. The brightness scaling is applied to each led as the colors are rendered to the PWM channel.
FYI - this is an adaptation of the [python scaling code](https://github.com/jgarff/rpi_ws281x/blob/master/python/neopixel.py#L148-L150) which is, in turn, an adaptation of the original Adafruit [arduino scaling code](https://github.com/adafruit/Adafruit_NeoPixel/blob/master/Adafruit_NeoPixel.cpp#L924). So I'm merely standing on the shoulders of my predecessors.