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
Report the iso path used by the vm to help debug possible support issues
Revert "Update default ISO path to prefer an ISO that's next to the boot2docker CLI binary"
Fixing name collision of driver.ErrMachineNotExist and virtualbox.ErrMachineNotExist
Moving ErrMachineExist to driver package
"machine already exists" is not virtualbox specific
Fixing error message when no suitable driver found

Right now The error message is misleading in case of a non existing driver: "boot2docker-vm": machine not supported
Its not the machine which is not supported, but the driver

    $ boot2docker --driver=nosuchdriver info

    Boot2Docker-cli version: v1.3.0
    Git commit: 1d0b71b
    error in run: Failed to get machine "boot2docker-vm": machine not supported
Merge pull request #289 from boot2docker/revert-283-boot2docker.iso

Revert "Update default ISO path to prefer an ISO that's next to the boot...
Merge pull request #288 from SvenDowideit/add-iso-path-to-b2d-info

Report the iso path used by the vm to help debug possible support issues
Starting from saved state skips setUpShares

Fixes #292
printExport support for fish-shell (based on boot2docker/boot2docker-cli#260)
Merge pull request #293 from lalyos/start-from-saves-state

Starting from saved state skips setUpShares
Make "socket" an alias of "shellinit" and hide it from usage
printExport always showing the export/unset commands
bash's behavior is default in printExport
refactoring printExport again
type fixed in printExport
getting rid of unnecessary variable attribution
Merge pull request #299 from tianon/no-more-socket

Make "socket" an alias of "shellinit" and hide it from usage
forgot to add value in the fish variable export, sorry