Added functionality to dynamically change applications

[dmccrevan]

I added a function changeConfig so that users could easily change the luis's configuration if the application is using multiple luis's workspaces.

Sample usage:

var LUISclient = LUISClient({
  appId: APPID,
  appKey: APPKEY,
  verbose: true
});

var newConfig = {
  appId: newAPPID,
  appKey: newAPPKEY,
  verbose: false
};

LUISclient.changeConfig(newConfig);

Update: In my second commit, I changed my code to more fit the way previously used to initialize the LUISClient

[msftclas]

All CLA requirements met.

[dmccrevan]

@Ahmkel Could this be merged in?