Add _.debounce to Underscore

[fschaefer]

I've extended Underscore by mixing in a debounce function from: http://unscriptable.com/index.php/2009/03/20/debouncing-javascript-methods/
and used it in conjunction with Backbone.sync to limit calls that may be happening several times because of async actions.

It would be nice to have this function in Underscore. Maybe also the throttling stuff:
https://github.com/cowboy/jquery-throttle-debounce/blob/master/jquery.ba-throttle-debounce.js

[jashkenas]

Good idea. I've added a _.throttle, and _.debounce, here:

9e3e067

... let me know what you think.

[cehoffman]

Awesome, can get rid of some code with this.

[fschaefer]

Works as expected. Thank you for inclusion!