Generate Coffeescript source maps

It would be great if we could get this working for debugging the resulting JS in Firefox. [Some](http://ryanflorence.com/2012/coffeescript-source-maps/) [links](http://tarantsov.com/WorkflowThu/source-maps-with-coffeescript-and-uglify-js/).

Currently we're reading the compiler output on stdout:

``` python
compiler = subprocess.Popen(command, shell=True, stderr=subprocess.STDOUT,stdout=subprocess.PIPE)
out = compiler.stdout.read().decode("utf8")
```

So we would need to switch to using temporary files in order to get the map file out, and then making sure it gets served up - or at least stored locally. And we would also need to think about how to stop that leaking with published sites.

Any thoughts?