Making scaling of pack layout optional.

In a zoomable pack layout, it may not be desirable to have the
layout scale, especially with padding.  This adds a method to the
pack layout that allows turning scaling off like so:

  var layout = d3.layout.pack()
      .size([100, 100])
      .scale(false);

The value of scale defaults to true so that the current behavior
remains the standard.

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This came up in the juju-gui project at https://launchpad.net/juju-gui  Using the pack layout, a complex environment with several services was rendering them too close together: http://atl.mjs-svc.com/scaled.png - by intentionally not scaling k, we get: http://atl.mjs-svc.com/unscaled.png

This is, of course, potentially only of use to us, so it's understandable if it doesn't fit within the scope of d3.  If there are any questions, feel free to contact me at matthew.scott@canonical.com

~M