User switching

I made this for practice—it's my first time experimenting with transparent elements.

Many thanks to Glenn Van Bogaert for drafting me!

- Sarah's picture is from her avatar pack. Grab it: http://www.sazzy.co.uk/2011/11/downloadable-pack-of-avatars-for-webpsd-mock-ups/
- Background: http://goo.gl/h3vJH