IE10+ compatibility

- fixed `dataset` missing in IE10
- fixed `dataTransfer.setData()` accepting only `'text'` and non-null value in IE10 and IE11

Example still does not work fully in IE10+, but at least it does not throw errors.

Also I've made a running example at http://jsbin.com/yicoluki/1/edit
Maybe it is worth mentioning in README