Follow up from GoToMyPC use case with ndt7-client-go

[stephen-soltesz]

• Remove notice from README.md #65
• Make ClientName and ClientVersion overridable at build time #71
• Provide example command to set main.go ClientName at build time. See for example: https://github.com/m-lab/ndt-server/blob/master/build.sh#L11-L18
• Try next available server on doConnection failure #58
• What are the set of errors returned by ndt7-client go?

[robertodauria]

After #71 is merged, building the client with these flags allows to set a custom ClientName:

CLIENTNAME=my-client-name

go build -ldflags "-X main.ClientName=$CLIENTNAME" ./cmd/ndt7-client