Improve browser bundling with Browserify/Webpack

This adds a `"browser"` field to the `package.json` file to hint to both Browserify and Webpack that they should _not_ include the data for all locales (just English) when bundling for the browser.

`"browser"` spec: https://gist.github.com/defunctzombie/4339901#ignore-a-module
We should probably avoid being too generic with top level properties in `package.json`, so as not to be future hostile to browser package managers.