Zipcar Inc. helped create the modern-day hourly car rental industry. Now it is becoming part of the mainstream car rental world. Avis Budget Group, the third-largest car rental company, agreed to buy the Cambridge company for $500 million and make it a subsidiary. The deal comes as Zipcar, which controls an estimated three-quarters of the North American hourly car rental market, faces increased competition. Zipcar has grown quickly since it was founded in 2000 but struggles to earn consistent profits. It said it would earn its first annual profit in 2012.