netconf_client: support multiple sequential tests

[makkrnic]

add support for chaining multiple tests in form of builder-like pattern

netconf_client.create()                                                
  .get(null, function(reply) {                                         
    console.log("Reply")                                               
    console.log(reply)                                                 
    console.log(util.inspect(reply, {showHidden: false, depth: null}));
  })                                                                   
  .get(null, function(reply) {                                         
    console.log("Reply")                                               
    console.log(reply)                                                 
    console.log(util.inspect(reply, {showHidden: false, depth: null}));
  })