Fix issue #343

It's impossible to capture the *initialize* event using `selectize.on('initialize', handler);` because `selectize` object is returned just after the event is triggered. Using `$input` object to trigger the  initialize event can avoid this and captures the event. For example:
```
var $select = $(selector).on('initialize', function() {
  console.log('init');
}).selectize(options);
```