Allow updating data-placeholder text with liszt:updated events

I needed to update the data-placeholder text dynamically, and it turns out it was a pretty easy one-line change. Simply call `this.set_default_text()` whenever the `liszt:updated` event is triggered.

You can see an example of the update here:
http://jsfiddle.net/timcheadle/rLmCQ/

This should fix issues #493 and #546