app: accept empty string in SetCName
api: add test to ensure that setCName accepts empty cname
cmd/tsuru: added unset-cname command
cmd/tsuru/developer: register unset-cname
cmd/tsuru/ops: register unset-cname
cmd/tsuru/developer: docs for set-cname and unset-cname commands
misc/bash-completion: add set-cname and unset-cname
cmd/tsuru/developer: update date in license header
cmd/tsuru/developer: version 0.6