Add Private Message count to User Dropdown

This adds an unread message count to the user dropdown, suggested in Sam's comment https://github.com/discourse/discourse/pull/1937#issuecomment-34951738  

*User with 2 unread private messages:*
![image](https://f.cloud.github.com/assets/3401659/2164193/af1e5714-94e8-11e3-9a78-1b9413ea9dd1.png)

*User has no unread PMs:*
![image](https://f.cloud.github.com/assets/3401659/2164159/32b3239e-94e8-11e3-91c5-28881d0ad6d9.png)

I also considered using {{i18n user.messages.unread}} so it would display (in English): "Messages (2 Unread)", but I'm not sure if that would translate cleanly. Let me know if we should user that or some other alternative wording.