xColor: Manipulate color values

This class can be used to manipulate color values.

It can perform several types of operations with colors like:

- Merging two colors with a given level the opacity
- Invert a color
- Complement two colors
- Add two colors
- Subtract two colors
- Multiply two colors
- Get the average between two colors
- Lighten a color
- Get parts of two colors
- Get the grey version of a color
- Get the Web safe version of a color
- Get a color for text to contrast well on a given background color
- Determine if a text color is readable on given background color
- Get the distance between two colors
- Get an array of gradient colors
- Get the gradient level of a given color between colors