Bower may suits your needs (1) and (2) for the rest you have requirejs.
From the readme :
Bower is a package manager for the web. Bower lets you easily install assets such as images, CSS and JavaScript, and manages dependencies for you.
To install a package:
bower install jquery
bower install git://github.com/maccman/package-jquery.git
bower install http://code.jquery.com/jquery-1.7.2.js
bower install ./repos/jquery
