It is a bit more complicated than grabbing a zip archive, but bower, npm, and git are standard tools for JS devs these days. now that you have npm and bower, you have nearly direct access to hundreds of js libs including Paper.js, Two.js, Raphael.js, D3, JQuery, Underscore... in-fact most reputable js libs will be hosted on bower and/or npm.
Additionally, you can keep your libs easily updated using these package managers
$ bower update paper
$ npm install raphael
The most important part of using these is that if you publish your own code, you don't need ti include dependancy libs like paper
. You can simply include a package.json that will allow whomever loads your code to install dependancies.