cmd/tsuru: change app-list so it displays the cname when it's defined

Closes #313.
cmd/tsuru: change app-info so it displays the cname when it's defined

Closes #314.
cmd/tsuru: add set-cname command

Related to #312.
cmd/tsuru-admin: register set-cname command

Related to #312.
cmd/tsuru/developer: register set-cname command

Closes #312.