When creating the `git-webkit pickable` command, we started to classify commits based on their commit message and content. As I've started working on various systems automating bug state as commits are landed, I've realized that the concept of "CommitClasses" is a more broadly applicable one. We should break out `git-webkit pickable`'s concept of "CommitClasses" into a seperate object and command, allowing systems to query a commit's class outside of `git-webkit pickable`.