use async.mapSeries for --slow installs

This is clearly a ridiculous pull request, and I don't expect it to be accepted. What I would like, however, is to call attention to the fact that during an install operation on a large project, the cpu and memory consumption of npm borders on obscene. I have several projects now that have too many dependencies (about 30 top-level items) to install properly in their production environments, because the environments are allocated according to the resource consumption of the projects themselves and not the installation process.

Rather than installing everything on another system, then moving the resulting files over to the destination system (which is basically what I'm doing now), I'd like it if I could tell npm to just cool it a little bit and maybe not go completely hambone trying to install everything it possibly can at once. This is a proof-of-concept of that idea, in that with this patch applied, and using the `--slow` install option introduced therein, I am able to perform a successful installation of the aforementioned projects in their production environments.

With this patch applied, the install time for browserify (chosen as a commonly-used project with a lot of dependencies) jumps from about a minute to over 15 minutes, which is obviously unacceptable for most use cases. For my case, it's perfectly fine, because there is a delayed, staggered install process anyway across multiple nodes, and this is just some additional latency in that process getting started.