Add support for upgrading binaries

`boot2docker upgrade` will support the Docker client binary,
boot2docker-cli binary, and boot2docker ISO now.

Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com>
Add small doc section on upgrade

Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com>
Implement a few things from code review feedback

Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com>
go vet

Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com>
Revise clobber flag default code

Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com>
Merge pull request #242 from nathanleclaire/upgrade_client_binary

Add support for client binary on upgrade