[SUREFIRE-1098] Fix runOrder=balanced is not working

Fix for #48 with IT.
Updated documentation for runOrder parameter.
Added ststem property for runOrder.
Andreas, but this field is protected. So can I duplicate it in Failsafe and SurefireMojo?
I needed this update because the IT called `.runOrder( "balanced" ).executeTest()` without any effect. Moving the runOrder to POM of IT worked fine but was not ideal solution either.