Exposes get_screenshot_as_file method.

You can now get a screenshot of the page being visited by using the get_screenshot_as_file method:

```python
In [6]: browser.get_screenshot_as_file('screenshot')
Out[6]: '/var/folders/c3/gg3q87ys51z86n43xgxrh7ch0000gn/T/tmpwmvYDwscreenshot.png'
```