Add models

This folder has all the models that are used for storing the API JSON data. For example, if an authentication API returns an ID and a token, the model file will look like this:

class UserData {
  int id;
  String token;

  UserData({this.id, this.token});

  UserData.fromJson(Map<String, dynamic> json) {
    id = json['id'];
    token = json['token'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['id'] = this.id;
    data['token'] = this.token;
    return data;
  }
}