Update Html Helper to include bootstrap icon generation.

Not sure where to put this. But its more so since bootstrap is built into yii framework. Have proposed this in ```BaseHtml``` class, since the bootstrap glyphicon is potentially used in a lot of places, and its a lot easier to use it like the following:

```php
echo Html::icon('pencil');
```
The prefix ```glyphicon glyphicon-``` and tag ```i``` or ```span``` can be customized as parameters.