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
```
Instead of special casing 0 and returning here, could you just check the type of amount and only set to 50 if it is undefined?  I'm assuming that will end up with the same result

    if (typeof amount === "undefined") {
      amount = 50;
    }