[Bars] Support for stacking datasets

Adds `stacked` to the plot options, set to `false` by default. When set to `true`, datasets are stacked on top of one another, rather than interleaved alongside each other.

![captura de pantalla 2013-07-09 a la s 13 03 51](https://f.cloud.github.com/assets/210225/769947/f5c5d2d8-e8c1-11e2-831c-11ff046e0049.png)

```javascript
var data = {
    labels: ['One', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven'],
    datasets: [
        {
            fillColor: '#4879B4',
            strokeColor: '#CCC',
            data: [60, 30, 25, 33, 24, 56, 78]
        },
        {
            fillColor: '#CC2568',
            strokeColor: '#CCC',
            data: [40, 20, 75, 12, 45, 67, 77]
        }
    ]
};

var ctx = document.getElementById('bars').getContext('2d');
var chart = new Chart(ctx).Bar(data, {stacked: true});
```