Multitouch pan and zoom support

[natevw]

see https://github.com/simplegeo/polymaps/issues#issue/39

I've ported the requisite code from my OL multitouch handler to Polymaps, and it's working rather well on my first-gen iPad and fourth-gen iPod touch. (Don't know much about what Android browser(s?) support; the webOS browser on my Pre 2 supports neither SVG nor multitouch DOM events.)

[shawnbot]

Yes, you should do this. :)

[natevw]

I've fixed both the TODOs that the wheel code had, in both the original wheel function and the copy-paste to touch.

As far as the second (container change) TODO that was in Wheel.js, the map itself doesn't seem to switch to a new container properly, so I'm not sure handling it was worth the extra code...

[natevw]

Please disregard previous patches — the touch branch I started with was many commits behind trunk. Now works with the WebKit bug44083 workaround, and imitates the more recent wheel.map setter (without the TODOs).

[rausanka]

+1 for having this pull request accepted

[mbostock]

Hey @natevw! Thanks for your work. I incorporated pieces of this and Tom's earlier "touch" branch in the latest 2.5.0 release. My implementation is similar to yours, although I decided to limit events to 1 (pan) or 2 (pan+zoom) touches. On the plus side, I added double-tap and rotation support, which is kinda cool. :) Let me know if you want to make any further changes to touch support.