Add Underscore Throttle to updateToolbar and provided option

We just started using bootstrap-wysiwyg for our site and some customers where complaining about our new HTML-editor was very slow. After some investigation, we discovered that the updateToolbar is called on every keypress which is very ineffective and results in performance problems when the users computers isn't very good.

This pull-request uses Underscores throttle to call the updateToolbar only every 200-millisecond and if you want to override it, you set the 'keypressTimeout' option to the number of milliseconds you want to keep it within.