Add driver#scroll_to and :selector option.

`driver#scroll_to` allows scrolling. For example,

    page.driver.scroll 200, 100

will set window.scrollX to 200 and window.scrollY to 100 if the viewport can
scroll that far.

The `:selector` option for render allows element capture.

For example,

    page.render selector: '#penultimate'

will take a screenshot of the entire element, even if it is outside the current
viewport.
Merge pull request #368 from jimjh/master

Add ability to scroll, and ability to capture a specific element.