Fix #191, #193, and add new setStyle function for more flexibility

Added showIcon option (set to true by default). While set to true, all
icons for their associated selected options are shown in the button.
When false, no icons are shown in the button. Added setStyle function,
which allows classes to be added/removed from the button and its
container.

The format is `setStyle(style, status)`.

If changing the style of the container (not data-style), use it as so:

````javascript
$('.selectpicker').addClass('span12').selectpicker('setStyle');
````

If updating data-style:

````javascript
// Replace Class
$('.selectpicker').selectpicker('setStyle', 'btn-danger');

// Add Class
$('.selectpicker').selectpicker('setStyle', 'btn-large', 'add');