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.
Merge pull request #283 from tianon/boot2docker.iso

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