Update default ISO path to prefer an ISO that's next to the boot2docker CLI binary

This is especially important for places like the Windows installer, where we want people to be able to install boot2docker 1.3 from an installer, and it includes the right ISO, so when they boot their VM, it should also be boot2docker 1.3, without any extra work or extra downloading.
I wonder if we should be doing an `EvalSymlinks` on this `os.Args[0]` first so that we can have this work for OS X too (without putting the ISO in `/usr/local/bin` or similar).