Dynamic optimizely script

Because we want to serve optimizely script, and possibly other scripts as well, from our own domain so we need a hack.

The idea is to have a custom script which pulls Optimizely API every couple minutes and check if JS script version changed. If it does, then fetch it, digest, save, and update `version` file keeping the digest (PR will follow).

This PR is about responsive site's part which is basically about fetching `version` file, building script tag, plus some caching to avoid hitting the disk on every request.

Thoughts?