Publish Cobertura report in buildDashboard

Added ability to publish Cobertura report in the Gradle build dashboard. Example show here:
![builddashboardcobertura](https://f.cloud.github.com/assets/696855/2046905/90c07674-8a0e-11e3-96e3-f6116c9ad817.png)

Enhancement was developed together with @aalmiray during #hackergarten.