remove unnecessary move_for_human() function
add human_wins() function

In order to test the canvas element, I need to check if the human can
ever have a winning position.  It returns 'good' or 'bad' and sets it
to the id attribute of a <p> element on the page (so that Capybara
can assert against it).
add add_game_over_element() function

This function draws an invisible <p> element to the page with an
id of either 'good' or 'bad', which will be checked against in
tests.