Compares two image given the threshold defined.

*Arguments:*
| filename - name of the image to be compared
| threshold - image difference threshold (optional with default value 0.0001)

*Example:*
| Navigate To        | http://jspringbot.org   |
| Screenshot Collect | jspringbot-homepage.png |

*jspringbot.properties*
| selenium.compare.threshold=0.0001
| selenium.compare.output.dir=/Users/jspringbot/workspace/jspringbot-selenium/target/robotframework-reports
| selenium.compare.base.dir=/Users/jspringbot/workspace/jspringbot-selenium/target/test-classes/imgbaseline/local-firefox/
