Add new options method. Possible fix for #78

I've added simple method to add new options into select.

User will have to take care about creating option element by himself.
i.e.:

{{{
// Init option
var option = document.createElement('option');
option.value = 55;
option.innerHTML = 'Test';
// Add it
$('some_element').multiSelect('add', option);
}}

Please notify if you'll want to put new option creation into multiSelect, don't think it's best idea however.