Add a getProperty() method for document element.

So we don't need to setup a hardcoded height and width for the canvas when working with processing.
For example:
```python
from processing import *

def setup():
    canvas = document.getElementById('mycanvas')
    canvasWidth = int(canvas.getProperty('clientWidth'))
    canvasHeight = int(canvas.getProperty('clientHeight'))
    size(canvasWidth, canvasHeight)
    # ...
```