Add the types property to the DataTransfer object.

This patch takes another step closer to functional parity with HTML5 drag & drop by adding support for the ```types``` property of the ```DataTransfer``` object. This ```types``` property is an ordered list of all the data types that were set with the ```setData``` method, in the order that they were first added.

See [types](https://developer.mozilla.org/en-US/docs/Web/API/DataTransfer#types.28.29)