Cleanup of Twitter Bootstrap for multi-column display

I like @drahmel's two column idea in #108. It's really fun to play with the "questions to consider" side by side with the crossfilter updates.

I've cleaned up that branch to remove superfluous whitespace changes which should make review easier. I also simplified this commit by removing the button attributes on the links, and separately recovered some of the margin. But otherwise this version ([viewable here](http://dribnet.github.io/crossfilter/)) should otherwise match [drahmel's upstream version](http://drahmel.github.io/crossfilter/) in #108.