Pass indexes in event params, add placeholderContents option, fix placeholderSize consistenty

Modified the plugin a little bit to suit my needs, might be worth incorporating:

1. pass `oldIndex` and `newIndex` as parameters when the `sortupdate` event is triggered. I use this as an easy way to update Backbone collections in response to changes in the UI.

2. add `placeholderContents` option to plugin for more customizable placeholders.

3. found a bug in how placeholder sizes were set, made them inconsistently sized. Fixed this.