Use POSIX::Spawn if available to speed up forking

This pull request adds support to use the posix-spawn gem if it is available (https://github.com/rtomayko/posix-spawn). This can greatly speed up forking processes when the application has a large heap by preventing the entire heap from being copied to the new process. This is most noticeable on Linux and can be quite pronounced when the heap is from a large web application.