Add various improvements

- Use 0 - 100 for color and alpha channels instead of 0 to 1
- Restrict colors to 0 - 100 for color channels
- Restrict hue by a 360 modulus
- Reverse blend/blenda percentage calculations (the spec has this reversed)
- Stringify to 10 digits of precision for percentages and alpha values (like Sass 3.5.0)
- Use big.js for more precise calculatons

Author: jonathantneal