refactor: move core package to com.qcloud.im folder (#6)

src/main/java/com/qcloud/im/IMClient.java

 package com.qcloud.im;
 
-import com.qcloud.im.model.core.Account;
-import com.qcloud.im.model.core.Message;
-import com.qcloud.im.model.core.Profile;
-import com.qcloud.im.model.core.SNS;
+import com.qcloud.im.core.Account;
+import com.qcloud.im.core.Message;
+import com.qcloud.im.core.Profile;
+import com.qcloud.im.core.SNS;
 import com.qcloud.im.util.SigUtil;
 
 import java.util.concurrent.ConcurrentHashMap;

src/main/java/com/qcloud/im/model/core/Account.java → src/main/java/com/qcloud/im/core/Account.java

-package com.qcloud.im.model.core;
+package com.qcloud.im.core;
 
 import com.qcloud.im.IMClient;
 import com.qcloud.im.model.request.AccountImportRequest;

src/main/java/com/qcloud/im/model/core/Message.java → src/main/java/com/qcloud/im/core/Message.java

-package com.qcloud.im.model.core;
+package com.qcloud.im.core;
 
 import com.qcloud.im.IMClient;
 import com.qcloud.im.model.request.ImportMsgRequest;

src/main/java/com/qcloud/im/model/core/Profile.java → src/main/java/com/qcloud/im/core/Profile.java

-package com.qcloud.im.model.core;
+package com.qcloud.im.core;
 
 import com.qcloud.im.IMClient;
 import com.qcloud.im.model.request.PortraitSetRequest;

src/main/java/com/qcloud/im/model/core/SNS.java → src/main/java/com/qcloud/im/core/SNS.java

-package com.qcloud.im.model.core;
+package com.qcloud.im.core;
 
 import com.qcloud.im.IMClient;
 import com.qcloud.im.model.request.FriendImportRequest;