auth: add parameter on GenerateToken to export in a tsuru app

It will use the private environment variable TSURU_APP_TOKEN.
cmd/tsuru-admin: added --export (or -e) flag to token-gen command
cmd/tsuru-admin: fix token-gen registration
cmd/tsuru-admin: using the export flag in the token-gen command
cmd/tsuru-admin: version 0.1.3