Use [Cliver](https://github.com/yaauie/cliver) for command-line dependency detection
Don't lazy-verify dependency; test clearer; update changelong
Merge feature/detect-phantomjs-with-cliver; Closes #366
update CONTRIBUTING doc to be easier to parse at-a-glance
Add driver#scroll to allow 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.
Add :selector option to allow element capture.

For example,

    page.render selector: '#penultimate'

will take a screenshot of the entire element, even if it is outside the current
viewport.