added highlight to element on hover by decreasing the opacity of the bac...

...kground elements using a global higlight function

@jebeck can you look at it, i think its ready but i'd like to hear what you think of the code and the resulting interaction.
Took me a long time to figure out why you were using this logic here. I'd love to make it more readable, and since again I want to argue that we need to avoid global `d3.selectAll`, I think we can just always pass the selection directly - either passing `d3.select(this)` (selecting `this` is the only exception to avoiding selecting from d3.select, and in fact I like to use it there since it makes it distinct)  or `mainGroup.select('.d3-wizard-1387234093')` as `selection` and then just `selection.attr('opacity', 1)` should do it.