Add Camo support

This pull requests adds optional support for [Camo](https://github.com/atmos/camo). Camo is a small application to proxy insecure assets over secure channels (mainly markdown images). For more information, check out the [wiki page](https://wiki.diasporafoundation.org/Installation/Camo).

The code might need some optimization, please review. Also I'm shamelessly using Travis to ensure my changes have no bad side effects, thus creating this PR. ;)
Little hack: `File.join(AppConfig.privacy.camo.root, digest, encoded_url` so we don't depend on the trailing slash in the config.