book_api.py change 1: searching

Pre-req: #106 (closed)

Search method should:

• query openlibrary's search api
• iteratively call parse_book_data(work_id) helper method to populate a list of book items to be returned

Conditions of satisfaction:

• Filter out any books that don't contain all required fields (those that returned null or equivalent from parse_book_data(work_id)
• Required fields: work_id, title, author, description, cover_id

must return at least 9 books, all with valid fields