Update scripts and docs to use yarn where appropriate

Most `npm` operations are replaced with `yarn`, which generally has better
behavior. However, steps like publish that write to the NPM registry are left to
`npm`, which currently handles these tasks best.

karma.conf.js

@@ -9,7 +9,7 @@ var webpack_config = require('./webpack.config');
  * to build everything; however it's the easiest way to load our dependencies
  * from node_modules.
  *
- * If you run karma in multi-run mode (with `npm run test-multi`), it will watch
+ * If you run karma in multi-run mode (with `yarn test-multi`), it will watch
  * the tests for changes, and webpack will rebuild using a cache. This is much quicker
  * than a clean rebuild.
  */