Better quality status images

Generated with

    wget -O status_image_success.png http://img.shields.io/badge/winbot-passing-brightgreen.png
    wget -O status_image_failure.png http://img.shields.io/badge/winbot-failing-red.png
    wget -O status_image_exception.png http://img.shields.io/badge/winbot-error-red.png
    wget -O status_image_warnings.png http://img.shields.io/badge/winbot-warnings-yellow.png
    wget -O status_image_skipped.png http://img.shields.io/badge/winbot-skipped-lightgrey.png

Fixes #1, better.