Add color support, a la Turtle!

- Adds Sphero::COLORS, a hash containing string colors matching to RGB values.
- Adds Sphero#color method that finds a color from this hash and proxies to
  Sphero#rgb.

Example usage:
  Sphero.start do
    color 'tomato' # equivalent to rgb(255, 99, 71)
  end
Merge pull request #4 from hybridgroup/colors

Adds color support, a la Turtle!