Use the jekyll-assets plugin

Using two build tools for asset preparation is awkward, especially since compiled assets are checked in to the repo. The jekyll-assets plugin will let me unify it into one, although I will need to do more work to fix the CDN upload plugin now.

I'll be fixing #19 while I'm at it.

The [README for jekyll-assets](https://github.com/ixti/jekyll-assets#how-to-use-jekyll-assets) is a useful reference.