Add touch support to `d3.svg.mouse`.

This doesn't quite seem like the right place for it, but it seems to work when
using "touchstart", "touchmove" and "touchend" handlers without any adverse
effects on "true" mouse event handlers.

See <http://www.jasondavies.com/apollonian-gasket/> to see an example of
combined touch/mouse handlers working with this change.

d3.js

@@ -3176,7 +3176,10 @@ d3.svg.mouse = function(container) {
     d3_mouse_bug44083 = !(ctm.f || ctm.e);
     svg.remove();
   }
-  if (d3_mouse_bug44083) {
+  if (d3.event.touches && d3.event.touches.length > 0) {
+    point.x = d3.event.touches[0].pageX;
+    point.y = d3.event.touches[0].pageY;
+  } else if (d3_mouse_bug44083) {
     point.x = d3.event.pageX;
     point.y = d3.event.pageY;
   } else {

src/svg/mouse.js

@@ -10,7 +10,10 @@ d3.svg.mouse = function(container) {
     d3_mouse_bug44083 = !(ctm.f || ctm.e);
     svg.remove();
   }
-  if (d3_mouse_bug44083) {
+  if (d3.event.touches && d3.event.touches.length > 0) {
+    point.x = d3.event.touches[0].pageX;
+    point.y = d3.event.touches[0].pageY;
+  } else if (d3_mouse_bug44083) {
     point.x = d3.event.pageX;
     point.y = d3.event.pageY;
   } else {