Adds tinycolor.mix()

You can now mix two colors by an amont in the range [0 - 100].  Based on [LESS mix()](http://lesscss.org/functions/#color-operations-mix).

```javascript
    var red = tinycolor('red');
    var blue = tinycolor('blue');
    
    tinycolor.mix(red, blue, 50); // purple
```
Yes, good idea - keep it consistent with the others