Adds Getty Images integration

As part of a new service integration, Getty Images is a great platform. The API isn't free,providing a limited amount of query per day (1000) with low-res and watermarked images. This integration is mainly made for paid accounts, with unlimited access and high-res images access.
To successfully integrate Getty Images API, some backend changes had to be made since the API requieres simple authorization (sent over a custom header value) and Auth2 process for allowing access to browse images. For this reason, GROAuth2SessionManager, an AFNetworking extension, has been added as a dependency for simplifying the process of authenticating against an OAuth 2 provider. Because GROAuth2SessionManager isn't yet updated to the latest release of AFNetworking, I had to downgrade AFNetworking to 2.2.1 for now, waiting to the owner to upgrade. This downgrade means less bug fixes in AFNetworking, and probably some installation incompatibility over cocoa pods for now, but should be fixed soon.
Whoever finds this integration interesting, let me know if it can be improved.