added option for custom dropdown caret html

As the title says I added the ability to specify the caret html as an option so that it can be replaced with something other than `<span class="caret"><span>`. I did this so that it can be used with other icon fonts.
See my working example: ![](https://cloud.githubusercontent.com/assets/4628721/3025809/4864847c-e00a-11e3-92f7-468c1e9d62fb.png).
```javascript
$('.selectpicker').selectpicker({
    caretHTML: '<span class="entypo-down-open-big"></span>'
});
```