proxy.py: Returns early if no request.
changelog: Adds note about PhantomJS timeouts.
phantomjs.py: Increases timeout to account for slowness on Travis CI.
processor.py: Uses key constants and sends scrollTo on PhantomJS remote.
options: Changes scroll default. processor: Changes viewport size, scrolling.

options.py: Changes --phantomjs-scroll default to 10.
processor.py: Changes viewport and page size to realistic sizes. Some
website's infinite scroll breaks if the viewport is larger than the
initial content size (_cough_, Twitter, _cough_). Scrolls to top of page
after scrolling to allow snapshots with fixed position banners to not
cover the content.