Allow access to Color class

This exports the `Color `class, so that users can create their own methods on the prototype.

It's backwards compatible and doesn't break any of the tests.

Example usage:
```javascript
var Color = require('color');

Color.prototype.averagedRgbArray = function () {
  return this.rgbArray().map(function (val) {
    return val / 255
  });
};

var color = new Color({r: 20, g: 60, b: 100});
console.log(color); // [ 0.0784, 0.235, 0.392 ]
```