Fix issue with PackageMaker not being bundled with Xcode.

PackageMaker is apparently not included with Xcode anymore. It's not included in the auxiliary package which can be downloaded here:

https://developer.apple.com/downloads/index.action

The script now assumes PackageMaker is installed either in `/Applications/PackageMaker.app/Contents/MacOS/PackageMaker` or, as before, `/Developer/usr/bin/packagemaker`.