Added basic models

Added 3 models. First, the user, which has the following categories: name(string) card_num(integer) fb_id(integer) email(string) age(integer) email_verification(boolean).
Second, the post, which has the following categories: description(string) price(integer) user_id(integer) category(string).
Third, the transaction, which has the following categories: post_id(integer) buyer_id(integer) seller_id(integer) card_num(integer).