Update Guardfile to include initializer specs

Update .gitignore to ignore tags file
Implement redis cache fetching method

`AgentT::Cache.cache_fetch_json` now works like
`Rails.cache.fetch`, with json serialization.
Refactor code and use redis as cache store

- Refactor code in tasks controller
- Extract `UserSummary` and `HandlingError` concerns
- Use redis for caching in `UserSummary`
- Add `render_404` method