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
Hmm, this method seems to add line_numbers. `sum` is initialized with - let's say 15 - and then the values from `num_branch_hits_for_statement_on_line(line_number)` are added :-(