Fix touch events.

This should take care of #31 and #37. It's a clean approach:don't try to separate users into those who have touchscreens and those who don't; just handle whatever events you get. For some reasons why this is the right thing to do, see http://www.stucox.com/blog/you-cant-detect-a-touchscreen/

I also ran into the coordinate-finding problem that is fixed in #41 by @bacinsky. I didn't notice his pull request until just now, so my commit also fixes that problem, in a different way. If you accept his pull request then mine will need to be modified (just get rid of the big `$.each`). If you take mine as is, then I don't think his will be necessary.

My fix is more code but it's more separated from the guts of tablednd, as a generic jquery touch event fixup hook.