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
```
Looking at the other methods it is done this way:

```javascript
        amount = (amount === 0) ? 0 : (amount || 10);
```

Should I keep it consistent and write this?

```javascript
        amount = (amount === 0) ? 0 : (amount || 50);
```