Add User Avatar Dropdown

This implements the user dropdown feature from this [discussion on meta](https://meta.discourse.org/t/user-avatar-should-have-a-dropdown-menu/8907).

Currently how I've styled the dropdown (desktop):
![image](https://f.cloud.github.com/assets/3401659/2149320/f74c0686-93f2-11e3-9196-8543e25315bd.png)

mobile: 
![image](https://f.cloud.github.com/assets/3401659/2149487/236bf63e-93f5-11e3-831b-4a26a18def44.png)

A few points:
1. Any suggestions for an alternative icon to use next to activity?
2. Any aspects of the alternative screenshots (below) to incorporate?
3. Any other feedback on the code, tests, or styling is appreciated! 


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*Alternative styling with a larger logout button:*
![image](https://f.cloud.github.com/assets/3401659/2149327/1f59a138-93f3-11e3-8c5c-7ca1d09627fa.png)

*Alternative styling with the width of the other dropdowns (I cut out some width in favor of a lower mouse-travel distance):*
![image](https://f.cloud.github.com/assets/3401659/2149330/3818c1cc-93f3-11e3-9a33-c10ae225b030.png)