Make alpha a property, fix cmyk alpha bug
Add contribute section in README.
Add subtractive color mixing, fix critical bugs.

Note: Wolfram Alpha interpretation of subtractive color mixing:
"clipped addition/subtraction in cyan, magenta, yellow, and black
channel respectively."

Bug Fixes:
- critical _alpha recursion problem
- apply float bounds to cmyk setter input

Also use zip for addative / subtractive mixing
README update