Release v0.3.1

This release makes the preferences library thread-safe, and fixes
a bug with how the labels on "pressed" vs "released" in the robot
inspector.  This is the first release after competition, and is
what we'll be using to teach the new programmers over the summer.

Also, I'm trying this release with an annotated tag instead of a
lightweight one, since that's the "proper" way to do releases in
git.  It seems github releases like lightweight better, but jgitver
likes annotated better, so we'll see how it goes.