Updates to OSX dmg packaging

1) By changing from zlib to bzip2, package size is decreased 10% from 69 to 63MB.

On tests on my computer, both files opened appropriately quickly. I think that the bzip2 package opens a little more slowly, but it's tough to tell for sure. In any case, neither one is unacceptably slow.

bzip2 is only supported on OSX10.4+. Since GCS is only supported on OSX 10.6+, this is not a problem.

2) This gets rid of the dmg and replaces it with scripting. This reduces the repo size by 7MB and eases maintenance.