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

@jwalton, you have reached a level of awesomeness that cannot be fully expressed by words. 

![awe](https://f.cloud.github.com/assets/339168/209230/28039022-8261-11e2-9215-b0fb4ca76f63.png)

Seriously, thank you very much!

These are the modifications I had to do in order to get source maps working in _Chrome 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.