Implemented PostgreSQL database with purchase request classes.

Shifted the responsibility of talking to the database to the purchase request repo class from the purchase request class
purchase request class now simply creates an instance of the class for accessibility purposes.
Added functionality to purchase request repo class:
1. creates requests table
2. drops requests table (for testing purposes)
3. creates a purchase request/adds to database.
4. checks_availability of the puppy breed requested in the purchase request.
5. view all purchase requests.
6. finds price of breed requested
7. updates the status and puppy_id of purchase request for when the puppy is sold
8. finds the breed_id of the given breed
9.'prettifies' the object returned from database for sharing purposes.
All tests have passed.