Introduce passive element behaviour

A DOM element can now be set to behaving passively: this means it cannot be dragged but it still reacts to forces.
### How to set an element to behave passively
Use the following data-attribute: `data-box2d-passive` = `"true"` in order to set it passive.
e.g.

```html
<div data-box2d-passive="true">Hello I'm passive</div>
```
will make this div passive