Buffer rendering using a document fragment for increased performance

This is a pretty minimal change with some significant performance improvements. Instead of doing the initial render/reset directly into the element, I use a document fragment to collect the items and then insert that fragment into the element in one shot.

Performance difference illustrated here:  http://jsperf.com/marionette-documentfragment-collectionview/5

This isn't a breaking change like #740 is, but it does have one questionable side-effect: People who have overridden appendHtml will not enjoy the performance improvements without some additional changes on their end.