Update Html Helper to include bootstrap icon generation.

Not sure where to put this. Since Twitter Bootstrap is built into yii framework - proposing this in the ```BaseHtml``` class. The bootstrap glyphicon is potentially used in a lot of places (in views, grids, navs, menus, buttons), 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. So this can work even if we have something like ```FontAwesome``` or  ```Jquery UI icons``` to be generated by changing the prefix.