Default Touch Context To Select Data Points

The initial call to event.preventDefault(); disables ipad 1 and ipad 2 and potentially othertouch devices from ever selecting data points using the default interaction model. This will allow selection of the closest point using a touch device on a single touch. It does not invoke the highlight callback like the usual Dygraph.prototype.mouseMove_.
Tested against ipad1, ipad2, Android 4.03 VM, Android 2.3.3 VM. using single line chart.