Inline strong parameters in CommentsController#create

...as there was no need to extract a method. The code looks more explicit now.
Add authentication and comments to page objects

...and put some decoration (e.g. data-role) on comment HTML, so that we can
recognize it.
Allow only signed users to see comments form

* Put specs for this feature
* Put specs for comments by signed users
Double quotes :disappointed:
Less specific selector for `assert has_comment?`

Thank you @exalted :wink:
Merge pull request #29 from catchup/clearer-auth-status

Clearer auth status