add sleep to decrease cpu usage while waiting for response

[vikoff]

before: $ time webkit2png --xvfb 1024 1024 -g 1024 1024 --scale 200 200 --aspect-ratio=keep --timeout=30 --log='' -o /tmp/img.png http://long-site real 0m10.553s user 0m8.383s sys 0m2.092s

after: $ time webkit2png --xvfb 1024 1024 -g 1024 1024 --scale 200 200 --aspect-ratio=keep --timeout=30 --log='' -o /tmp/img.png http://long-site real 0m11.074s user 0m0.412s sys 0m0.078s

[ghost]

Isn't 0.5 seconds quite a lot?

#62 also has this change: 81460b13e5a386cbffca52fea1ec293b7c0f06aa