Moved folder structure

This is a v2.0 milestone to create a more consistent directory structure for the project as described in:  https://github.com/mozilla/metrics-graphics/issues/262

There were no actual code changes, however, paths were updated to work with the new separate folders.  In the demo files I removed a path to metricsgraphics-layout.css that has a 404 in production.

I also created specs, gulp, and scss folders.  Those will be updated in future tickets.

I made a couple of small additions to the package.json to allow for future npm use.  As well as an index.js file.  That will be valuable if we can migrate the library to be commonjs/amd compatible.

Let me know if you want me to set the PR to a v2 branch or anything...

Hope it helps.