v0.1.0

Implemented REST API endpoints:

1. Create a new product
2. Get all products
3. Update product data
4. Create a new fixed-amount promo code.
5. Create a new percentage promo code.
6. Get all promo codes.
7. Get one promo code's details by providing the promo code text.
8. Get the product discount price by providing a product name and a promo code text.
9. Simulate purchase