Back button

[sethwoodworth]

We need to have a clear button to unzoom the d3 pack layout.

[btbonval]

In branch feature_d3_zoom, zoom out can be done with scroll wheel pretty intuitively.

Otherwise, this might be handy if it were more illustrative:
http://thisismattmiller.com/blog/add-zoom-slider-to-d3-js/

My guess: it's just a rectangle that the user clicks in. Top of the triangle zooms in, bottom zooms out. Prettify up the rectangle with some triangles or something and it should be a good secondary zoom feature.