Allow users to tag other users in an accomplishment or comment with an @

[rohankapoorcom]

For example: 'Migrated Pulse from Nodejitsu to Heroku with @kbng2.' should show 'Migrated Pulse from Nodejitsu to Heroku with Ben Ng'

[rohankapoorcom]

Similarly, we need to display the user's netid when you hover over their name

[ben-ng]

I would like to see this too. While push notifications would be nice, it's sufficient to send an email if the mentioned user is not currently online.