Add "nproc" support to getrlimit/setrlimit (for supported OSes)

This picks up on @fbrusch's pull request from #22. It adds the `nproc` support, but is `#ifdef`ed like you suggested, and I updated the documentation in the README.

I also added tests that I believe should work across platforms to ensure that it's supported on the expected platforms but still raises errors on unsupported platforms. I don't have access to a Solaris or Windows box to totally verify these tests across platforms, but from what I gather it's only supported on Linux and BSDs (and they pass locally on a Linux and OS X box), so I'm hoping the test logic for platform support is correct.