Debounce resize events to improve performance

When using more than a couple fluid placeholders at a time,
the performance of resizing the browser window can degrade
significantly.

To fix this I added a "debounce" function to the resize event.
Basically it only calls resizable_update() if a certain amount
of time (100ms by default) has passed since the last time the
browser window's size changed.

The debounce time can be changed via Holder.run({debounce:<time>});
Make if falsey to disabled it completely.
Merge pull request #96 from msealand/debounce

Debounce resize events to improve performance