As a user, I want to be able to direct comments at particular people and have them get notification of that comment.
(Here is an example of what our competition is doing: http://help.osf.io/m/collaborating/l/524111-commenting)

Acceptance Criteria:
1. In the comments section, users can address their comment to a particular person by typing @ followed by the person's name.

2. As the commenter types the name, a list of available people pops up from which the commenter can select. (note: might get confusing if the user has two accounts with the same name--how would the commenter know which to select?) The list of available people should only include those who have access to viewing the object... in other words, members of the project space and people outside of that space only if the object is public.

3. The mentioned person sees the comment as an event in their event feed and also receives an email that contains both the comment and the link to the object being commented on.