Use image.bash to construct a disk image.

package.bash constructs a package file (Go.pkg) for installation on OS X, and
is used by image.bash to construct a disk image. Strictly speaking, the disk
image is unnecessary, but they are more common by convention.

These scripts depend on PackageMaker (Developer Tools), osascript, and hdiutil.
Appropriate checks are run in utils.bash, called at the beginning of each
script.