Add User Profile Picture support

[maexbower]

Some users told me, that it would be great to see an image of the person when hovering over the list, so they can see who is in their shift.
This surely is a nice to have, but I guess it would make it a lot easier for the new volunters to assign to their first shifts.

This could be automated when people are connected with facebook https://developers.facebook.com/docs/graph-api/reference/user/picture/#Overview

Although it would be cool if everybody has a picture, I guess it have to be up to the users to set one or not.