Use absolute path in JSLint XML reporter

Hi, we're using JSHint with Jenkins' [Violation](https://wiki.jenkins-ci.org/display/JENKINS/Violations) plugin. Recently we just upgraded JSHint from 0.7.1 to 1.1.0 and we cannot view per-file violations anymore(clicking filename gives a blank page).

It seems the only difference is that JSLint reporter used to print absolute paths and now it's all relative. Thus this patch.

This patch is by no means a ready-to-merge one, just a rough start to tackle the problem.