Coverage Service for Cobertura XML

Hi marklarr,
as promised here is the pull request for the Cobertura XML coverage service.

It generates a nice xml report conforming to the dtd ([see here](http://cobertura.sourceforge.net/xml/coverage-04.dtd)) including line and branch coverage.

Have a look at it and tell me what you think.

Cheers
tarbrain
We can make these changes in `CoverageFile`'s `gcov_data` method so `CoverallsCoverageFile` can benefit from some of the additions here, like cleaning the gcov data. :+1: