Cumulative emission total on GHG emissions chart

I beg your pardon but I didn't know how to submit a merge request for a single file. There is only one file that you might care about in this request, which is:

    src/javascripts/primary_energy.js.coffee

All the rest of the changes are just compiled assets which (now that I've tried merging from upstream and submitting this pull request) I think would be better off being excluded from the repo.

The change in question is a refinement of a hack to add cumulative carbon emissions to the GHG emissions chart, made at the recent Manchester hackday (25/11/2013). You can see it in operation here...

    http://2050.hellings.webfactional.com