Fix typo in functional test comment.
Use onUrlChanged to detect page navigation.

 - http://phantomjs.org/api/webpage/handler/on-url-changed.html
 - Increase PhantomJS min version to 1.6.0.
 - Replaces polling triggered by onLoadStarted.
Add missing err argument to getPathname callbacks.

The callback for `page.evaluate` should expect an error argument
followed by the result. Our callbacks were expecting the result only.

This means our assertions were checking that null === null instead of
checking if pathnames matched. Both assertions pass, but the latter
is more useful for finding bugs. :)
Fix #79. Do not lookup agents when destroyed.