* Change how blockly handles cursors. The old way was quite slow because it changed the stylesheet directly. See issue #981 for more details on implementation and tradeoffs. This changes makes the following high level changes: deprecate Blockly.Css.setCursor, use built in open and closed hand cursor instead of custom .cur files, add css to draggable objects to set the open and closed hand cursors.
* Add a workspace drag surface that blocks and bubble get moved to during a workspace drag.
The surface is translated using translate3d instead of svg's translate attribute so that
the browser does not have to repaint the entire workspace on every mouse move.
This is very similar to the block drag surface.
* Address code review comments
* add back hasClass_ utility removed in #748 and stop using contains since it is not supported in IE
