Serve and build images/fonts from Sprockets gems, and properly integrate Compass/Sass with Sprockets

Wow, this took a lot of digging. Initially I was looking for a solution to #19, but I realized that #13 was a prerequisite (supporting images from asset gems). I took @jordanandree's work and built on it until I was happy with the behavior.

Now, all images/fonts from Sprockets gems will be copied into the output. I'm pretty sure you can still `ignore` and `proxy` them, and deeply nested assets preserve their structure. They're all real Sitemap entries. And it all works with @asgeo1's relative build directory outside the root, in build and in preview.

Fonts won't actually work 100% until I make a small change to `asset_path` in the main Middleman repo.