cmd/client-api: exit status 1 when HTTP response code is >= 400

cmd/client: added "-user" to add Basic user:pass authentication

cmd/client-api: show User-Agent saying who we are

cmd/client-api: added option "-stdin"

cmd/client-api is finished for POST and PUT methods

Added Client.WS to connect using websockets

Client.Request: if dest is a pointer to a []byte, it will receive the…

… output as-is

added api.Output() to send arbitrary JSON-encoded values

Handler: allow empty body even when expected an input

Added OutputWithStatus()