General improvements

* Add `Guardfile` in order to user guard in development
* Add [rubocop](https://github.com/bbatsov/rubocop) as development dependency
* Make file linter friendly
* Improve tests and code for `CommentsController`

As promised I reviewed this. Instead of writing text I decided to write real code, so this is just an example in how to proceed with your code and how to structure it. Keep in mind these are general patterns and not the truth for all your code.

BTW, Is the project still in development?