Re-export skygear-core classes in skygear module

[limouren]

Currently there is a list of classes accessible as attribute accessor from the default export of the skygear module:

import skygear from "skygear";

skygear.Query;
skygear.Role;
skygear.ACL;
skygear.Record;
skygear.UserRecord;
skygear.Sequence;
skygear.Asset;
skygear.Reference;
skygear.Geolocation;
skygear.Database;
skygear.Friend;
skygear.Follower;
skygear.Following;
skygear.Error;
skygear.ErrorCodes;
skygear.AuthContainer;
skygear.RelationContainer;
skygear.DatabaseContainer;
skygear.PubsubContainer;
skygear.PushContainer;

It would be good to be able to import these classes with the new es6 module syntax:

  import skygear, {
    Query,
    Role,
    ACL,
    Record,
    UserRecord,
    Sequence,
    Asset,
    Reference,
    Geolocation,
    Database,
    Friend,
    Follower,
    Following,
    SkygearError,
    ErrorCodes,
    AuthContainer,
    RelationContainer,
    DatabaseContainer,
    PubsubContainer,
    PushContainer
  } from "skygear";