Implement System Coverage Metrics

Hi Sonar devs,

Ive implemented the system coverage metric in sonar core, as per https://jira.codehaus.org/browse/SONAR-4030

Please let me know if this can be included in the version 3.6? 

One thing needs to be checked, the coverage in new code for some reason was not showing, i tried also without this patch and there was no new coverage. Seems to be some regression on 3.5 that needs to be confirmed

Thanks in advance 

Jorge Costa
![SystemCoverage](https://f.cloud.github.com/assets/1041352/148512/82b1ec1c-7505-11e2-824e-f07dd994472b.png)