Improve page end condition

Now a page is considered done when all of the following messages arrive:

- 'Page.domContentEventFired';

- 'Page.loadEventFired';

- the respone of the original request.

This:

- actually reverts 859995ed3fa49067d6e772504f28cf98d22a7f30 since
  'Page.domContentEventFired' is not optional anymore;

- fixes #2 where 'Page.loadEventFired' arrived before the response of the
  original request;

- fixes #9 where 'Page.loadEventFired' arrived before
  'Page.domContentEventFired'.
Bump version