I recently ran `npm install` (npm 1.4.3) with the `--save-dev` flag and the package entries it added to my package.json all began with a `^`, e.g. `"^2.5.0"`. I have never seen this before with the earlier versions of npm I have used, and I cannot find any documentation for this notation, only for the notations I'm already familiar with, e.g. `~`, `>=` etc.. What does it mean?