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
```
Done.

This also raises a larger question about ensuring valid params.  How would you like to handle this in general?  For instance, do you want to check that the required params exist and that their type is also correct?  Then, if either of those are invalid, throw a helpful error?