Add a drag-and-drop helper

The helper is a Command wrapper that adds `dragFrom` and `dragTo` methods that generate JavaScript events to simulate HTML5 drag-and-drop. They use the current context element, or can be passed an element. Basic usage is:

```js
this.remote
    .findById('source')
    .dragFrom()
    .end()
    .findById('.target')
    .dragTo()
```

It's based on [HTML5DragAndDropSimulator.txt](https://code.google.com/p/selenium/issues/detail?id=3604#c58).