started support for OS X Yosemite

I started support for OS X Yosemite. I have downloaded the `InstallESD.dmg` from a Mavericks VM as [described here](http://stefanscherer.github.io/unboxing-os-x-yosemite-for-packer/) and adapted the Makefile and templates as far as I could.

I prepared the ISO and built an Yosemite desktop with

```bash
make dmg/OSX_InstallESD_10.9_13A603.dmg
make osx1010-desktop
```

But after packer boots the VM there shows up an error

**Failed to open OS X Installer.** 
The path (null) appears to be missing or damaged.

@timsutton Perhaps the `prepare_iso.sh` has to be updated first. Any ideas?