makes svg text non-selectable

A common pet peeve of mine with d3 is that when clicking around a chart, you'll often unintentionally select svg text, when really there is no need. So... I added some css so you can't actually click/select chart text. 

I'm going to see what I can do about getting mbostock on board with changing some of his examples, but I thought this was a good first step.

This especially improves usability on the rowchart, as you have to click on the actual bar and not the label in order to filter on it.