Proof of concept jscoverage support

Hiya, I was looking for a way of getting code coverage working as part of our CI build. This feature needs some work, but I wonder if you're interested in supporting code coverage in this gem?

The argument could be made for a separate gem, except that (a) it's a tiny amount of code and (b) as it is generated by the tests actually running it either needs to hook in deeply in this gem or run the tests twice which would be annoying.

Please let me know what you think, if you're generally supportive of the idea I can put some more work into this feature.

This uses the jscoverage binary from http://siliconforks.com/jscoverage/