Only attempt multiprocessing if Process.fork is available
Avoid nil count for in_processes, and use 0 if forking is not possible
JRuby threads seem to work fine, and should be default for this platform
More accurate processor core counts for Linux and Windows
More accurate and comprehensive processor detection and counting
in_processes: avoid count of 0, let map() handle fork() support
Removed host OS override test. Does not work with heuristic detection.
Merge pull request #80 from yaoguai/master

More robust platform support