Adding Cobertura coverage

First I hereby grant you license to reproduce whatever portions of the code inside this pull request (of course if you do so acknowledgement is always welcome).

I found myself recently in need of having the coverage report for iOS in Sonar so I tried my best to replicate the sonar-cxx approach.

The iOS project can produce the data and then postprocess the gcov formats with gcovr to obtain the cobertura.xml.

Since this is the first time I do anything on a sonar plugin I expect I got a few things wrong. Your comments and feedback are welcomed.