Only strip leading slashes from filenames when generating Cobertura reports

As described in issue #39, the leading slash in filenames is removed to ensure compatibility with Jenkins.

This works fine in web server mode when there is _always_ a leading slash present, but if you are converting a JSON coverage file that has been generated in file system mode (as per issue #48) then there are no leading slashes and the initial character of the filename is removed blindly.

This patch will only remove the initial character in a filename if it's a slash.