Support OSX pkg creation on MacOS X

add osx packagemaker support (keyed off OMNIBUS_PKG_NATIVE environment variable).  if the environment variable is not set, it simply performs the default action (build a tarball).

unfortunately apple has stuffed this up a bit and not only does it require xcode 4 but also requires xcode auxilary tools package to be downloaded and then packagemaker.app has to be copied from the dmg to /Applications/Xcode.app/Contents/Applications.