I simplified the implementation to only support single-touch. I also opted not to include touch support in the force-dynamic example (and other examples), just because it complicates the examples too much. Touch is nice but I don't want it to interfere with people learning the basics. This merge also has an extra bonus fix: you can now have multiple force layouts with dragging on the same page, and the drag behavior will do the direct the event to the appropriate force layout.
This allows the Dorling/Demers cartograms to be slightly closer to the real geography. Also, fix the Dorling collision detection as self-collisions were previously being detected. Thanks, Mike! Lastly, I've removed the variable per-link distance as this is no longer needed.
Also, rename force.distance to force.linkDistance, but keep force.distance as an alias for backwards-compatibility. Fixes #211.