Display group cumulated response time
Merge pull request #26 from excilys/group-stats

Display group cumulated response time
Minor clean up
Upgrade Highcharts 3.0.4 and Highstock 1.3.4, close excilys/gatling#1335
Upgrade Highcharts 3.0.5 and Highstock 1.3.5, close excilys/gatling#1345