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
I think this should be in the gemspec and not the Gemfile, otherwise it won't be included when people `gem install slather`.