Which framework are you using on the server side? Most of the frameworks out there come with an "asset pipeline" already built in or as a plugin.
For instance django has https://django-pipeline.readthedocs.org/en/latest/ and grails has http://grails.org/plugin/asset-pipeline
This does everything you asked about and more. I'm sure there's something similar for whatever you're using on the serving back-end.
Edit: to clarify - I don't think this is something people do manually. They have a tool which does it on the fly or at build/deploy time.
