Source maps generation in Makefile

jQuery got source maps in the recent 1.9 release, so I figured they must be reasonably stable and well-supported.

This change modifies `make min` to generate a source map. Unfortunately, uglifyjs assumes it's run in the output directory when generating source maps, so I had to hack the Makefile a bit to make this happen.

It would be really nice if less.js 1.4.0 had source maps, so I hope you will consider this change. Also, I'd be happy to add source maps to the recent releases in dist/ in a separate pull request, if that is desirable.