support for osx

stock osx comes with old bsd utils so /usr/bin/tar doesn't accept the
--concatenate option. luckily though stock osx comes with
/usr/bin/gnutar which is pretty much the same version as the gnu/linux
tar which accepts the --concatenate option.

if we're on osx (ie uname => Darwin) use gnutar rather than tar.
Merge pull request #8 from joshfire/master

support for osx