change gamma correction related things

Add Led.GAMMA_TABLE_64 and fix Led.GAMMA_TABLE_128. The Piglow class now
also has GAMMA_{32, 64, 128, 256} for easier use (no need to import
piglow_spark.components.led.Led, piglow_spark.piglow is enough).

Add the static method Led.gamma_correct().

Provide both auto gamma corrected and “raw” versions of the light
and buffer functions in relevant classes:
- light() is now “raw” in the Led and LedSet classes.
- buffer() is now “raw” in the Led, LedSet and Board classes.
- Board class: all(), led(), arm(), ring(), color(), led_set() and
  buffer() are now “raw”.
- the auto gama corrected versions of the functions are prefixed with
  "gamma_" (for instance, Led.light() and Led.gamma_light() or
  Board.arm() and Board.gamma_arm()).

PiGlowCmd now also provides commands for both “raw” and auto gama
corrected functions.
Merge branch 'internals' into dev

Improves gamma correction handling.

Changes the API.
bump v0.3.0