def add(key, value, ttl=nil, options=nil)
      perform(:add, key, value, ttl_or_default(ttl), options)
    end
def add(key, value, ttl=nil, options=nil)
      perform(:add, key, value, ttl_or_default(ttl), options)
    end
Conditionally add a key/value pair, if the key does not already exist
 on the server.  Returns truthy if the operation succeeded.