Display unread count badge in menu specific to the current user

Mostly resolves #588.

@lukecarbis @shadyvb please review.

1. I chose to only tackle the first item in the list. The others will need to be scoped out separately.
2. There is a user profile option for this. It's enabled by default. The unread count is specific for each current user.
3. The unread count _does not_ affect the current users' changes, only changes done by others.
4. If there are more than 99 unread records, it just displays as 99+. This allows us to limit the update query by 100 records.
5. The user-specific unread count is stored in a transient with a 5 min TTL.

![screen shot 2014-11-13 at 1 03 51 am](https://cloud.githubusercontent.com/assets/522158/5024777/1a699570-6ad5-11e4-8a81-d2c6beae3c5b.png)

![screen shot 2014-11-13 at 1 02 50 am](https://cloud.githubusercontent.com/assets/522158/5024783/378cfe58-6ad5-11e4-8c3a-a585d248378b.png)