Disable browser selection on drag examples

[MrNice]

Minor inconvenience: working on the drag exercises, one often accidentally 'selects' (as in text highlighting) a large portion of the viewport, as well as the div itself. This isn't a problem but it's annoying.

It would be nice if this didn't happen.

I'll work on a PR later, but here's what I'm using to guide me, if anyone wants to beat me to the punch: http://stackoverflow.com/questions/826782/css-rule-to-disable-text-selection-highlighting