support for updating UI after changes made to the select element

Example:

// add a disabled attribute to some options based on the value:
var select = $(mySelectElement);
select.find('option[value^="p-"]').attr('disabled', 'disabled');
// refresh the ui
select.selectpicker();

select.selectpicker();

N.B.

The previous pull request ( https://github.com/silviomoreto/bootstrap-select/pull/88 ) had messed up formatting, not showing the changes clearly. This one should be a lot better. My appologies for the mess :)