Fixed key name in source map, added coffee file to map sources

These are the modifications I had to do in order to get source maps working
in 27.0.1425.2 (Official Build 185250) canary. I haven't tested other
browsers.

I first looked at the V3 spec and a few examples, and I saw that the
`source` key of the source map should be called `sources`.

After doing the `source` to `sources` change, the coffee source and for
some odd reason the javascript file would not show up in the browser
dev tools (it was being fetched but not evaluated).

To fix this, I had to add the coffee source to the `sources` list in the
source map file.