Implement "Reply to..." and mentions functionality

[w32blaster]

As an user I want to reply to someone's comment. As an comment author I want to see all the replies to my comment.

DoD

To be implemented:

• Add field "replyTo" to the Comment domain
• Add event listener for any "Reply to..." button. When user clicks this button, the "add new comment" form appears and it is somehow marked, that the new comment will be replied to an existed one.
• Make it possible to select all the comments, that are replies to a given user
• Implement mentions. If someone creates a new comment and any name starting with @ is presented, then this person receives a notification (see link in the comment)

Technical todo:

• [x] Create new TagLib that could render comments in hierarchy. All the replyes must be displayed under its parent comment and with small indent
• [x] Imporove comment rendering: all the emails should be replaced with appropriate name. On the cursor hover a small popup should appear with real name and user avatar
• [x] Create new section "Replies" in the "Comments" page. An user must select all the comments, that were posted directly to him
• [x] Add notification about this event (User replies to another user)
• [x] Add Mention JS library

[w32blaster]

Add also "mentions" functionality https://github.com/jakiestfu/Mention.js Demo: http://jakiestfu.github.io/Mention.js/

[w32blaster]

This library suits better for our needs: http://podio.github.io/jquery-mentions-input/