Additions :

• Persisting access token using UICKeyChainStore. UICKeyChainStore is added as an optional sub-spec.
• Discontinued support for iOS 6, to comply with AFNetworking's compatible SDKs.
• Added - authorizarionURL helper method to InstagramEngine to generate Authorization URL with basic permissions.

Changes :

• - extractValidAccessTokenFromURL renamed to - receivedValidAccessTokenFromURL
• - getUserDetails: withSuccess: failure: accepts a string value for userId to maintain consistency.

Fixes :

• Call super encode/decode on subclasses of InstagramModel to keep ID property persisted
  Implemented by @natan in #140