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
branch_coverage_data.keys.reduce(0) do |sum, line_number|
  sum += num_branch_hits_for_statement_on_line(line_number)
end