Plugin: Rake-fast: Support both *nix and Darwin

Syntax for stat is different on *nix and Darwin (BSD)

BSD syntax: 
http://www.freebsd.org/cgi/man.cgi?query=stat&sektion=1&n=1

Darwin Syntax:
https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/stat.1.html

*nix syntax:
http://linux.die.net/man/1/stat

This pull request makes rake-fast work on both *nix and bsd platforms.