Use relative paths, print the line source more robustly

Couple of improvements, the first being relative paths--this lets you much more easily serve HTML covershot reports from the web, or via a quick `python -mSimpleHTTPServer`. I develop remotely on a Linode box and thus don't have the coverage report on my local system. I also run covershot on our Jenkins instance and the relative paths are needed for web viewing there as well. It should not affect local filesystem viewing.

The other improvement looks to be a quirk in some versions of `jscoverage`--my version writes its source output as `{ "3": { source: "source of the line here" } }` whereas it looks like covershot expects it to be in the format `{ "3": { line: "source of the line here" } }`, my change will accept either variant and so does not proscribe a specific version for the end user. (I'm using the one at [visionmedia/node-jscoverage](https://github.com/visionmedia/node-jscoverage), FYI)