Asset deploys

I've deployed this app and made a few tweaks to make it play nicely with the environment I'm working in.


- Added the ability to set config.action_controller.asset_host with the ENV['ASSET_HOST'] value.

- Moved the assets gems out of the special 'assets' group so that precompiles can happen locally and then have the files moved to a CDN. This also seems like the Rails 4 way of doing things.

- Added a staging.rb environments file that's an exact copy of the production.rb file.

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One other change I had to make to get it deploying was adding the rainbows gem to the Gemfile, but I don't want to propose that for master since I don't know how it will affect other people's deployments. I see the chef production recipe has a different Gemfile it uses.