Fixed issue regarding Bar charts and negative values.

This pull request should fix problem with Bar charts having negative values in its datasets.

I've also updated the example samples/bar.html for a demonstration of the fix.

When wanting to make use of this feature it's important to set the option scaleBeginAtZero to false:

```javascript
new Chart(ctx).Bar(barChartData, {
  scaleBeginAtZero : false
});
```