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
Do we need to be concerned about divide by 0 here? with `to_f`, it actually returns infinity

```ruby
2.1.1 :001 > 2 / 0.to_f
 => Infinity
```