Add support for the Element.classList interface

The classList interface has been available since IE10, and is quite widely available on modern browsers. Using this provides a hefty performance increase when highlighting large documents.

My own anecdotal evidence for the performance impact is that the processing time spent within rangy on a private application has dropped from 3.2s to 1.3s with just this patch.

# Update
I had to adjust the original patch as add/has/removeClass functions all can take white-space separated lists of classes. Fortunately, accounting for this, using the classList interface is still significantly quicker on large documents.