Fix comet length and simplification

[rhooper]

Fixes #27 and addresses other concerns in comet. Also adds an intensity calculation helper to color.

[dhalbert]

Nice reduction in code size!

[rhooper]

@dhalbert I added a note about why the code is the way it is.

[dhalbert]

Thanks. There's a little pylinting to do:

Run pylint $( find . -path './adafruit*.py' )
************* Module adafruit_led_animation.animation.comet
adafruit_led_animation/animation/comet.py:51:0: R0902: Too many instance attributes (13/11) (too-many-instance-attributes)
adafruit_led_animation/animation/comet.py:51:0: R0902: Too many instance attributes (13/11) (too-many-instance-attributes)
adafruit_led_animation/animation/comet.py:51:0: R0902: Too many instance attributes (13/11) (too-many-instance-attributes)
adafruit_led_animation/animation/comet.py:51:0: R0902: Too many instance attributes (13/11) (too-many-instance-attributes)
adafruit_led_animation/animation/comet.py:51:0: R0902: Too many instance attributes (13/11) (too-many-instance-attributes)
adafruit_led_animation/animation/comet.py:51:0: R0902: Too many instance attributes (13/11) (too-many-instance-attributes)
************* Module adafruit_led_animation.color
adafruit_led_animation/color.py:85:4: R1705: Unnecessary "elif" after "return" (no-else-return)