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
```
Agree on that.  This line is copy pasta from less.js.  Didn't test these alternatives, but is anything like one of these more acceptable?  Could give it more thought if you feel it's necessary.

```javascript
    var w1;

    if (w * a == -1) {
        w1 = (w + 1 ) / 2;
    } else {
        w1 = ((w + a) / (1 + w * a) + 1) / 2
    }
```
or

```javascript
    var w1;

    if (w * a == -1) {
        w1 = w;
    } else {
        w1 = (w + a) / (1 + w * a);
    }

    w1 = (w1 + 1 ) / 2;
```