fix pie chart callbacks #2

Events mentioned in config are as follows:
```
var config = {
  mouseover: function() {},
  mouseout: function() {},
  click: function() {},
}
```
whereas what is actually expected using pie chart is `onmouseover` and `onmouseout` instead of `mouseover` and `mouseout` respectively:
https://github.com/chinmaymk/angular-charts/blob/master/src/angular-charts.js#L623
https://github.com/chinmaymk/angular-charts/blob/master/src/angular-charts.js#L634

`click` seems to work just fine:
https://github.com/chinmaymk/angular-charts/blob/master/src/angular-charts.js#L641