Added support for bar labels in bar chart

Added support for showing labels(the data value) on top of the bars. By default, this is disabled and enabling this via "options" will bring the bar labels to the chart.

#### Sample code to bring bar labels.

```
var options={showLabelsOnBars:true,barLabelFontColor:"gray"}
var myLine = new Chart(document.getElementById("canvas").getContext("2d")).Bar(barChartData,options);
```
![github-chartjs-screenshot](https://f.cloud.github.com/assets/144469/1525191/e9989f22-4bd1-11e3-907a-af2791cfd6f8.png)