Move preventDefault to gesturemovestart (Fixes #1716)

Android browsers require that we prevent the default browser action on the start of drag. We tried this in #693/#1557, but this broke in other ways for other browsers.

The issue stems from preventDefault being called on the document instead of the Node.

This commit moves the preventDefault to the `gesturemovestart` in the prototype for `DD.Drag` so that the page is still scrollable.