An attempt to fix https://github.com/awslabs/aws-sdk-ios-samples/issues/...

This fix seems to plug the leak described in https://github.com/awslabs/aws-sdk-ios-samples/issues/4.

N.B. The fix covers only the listing of S3 buckets. Same problem is present elsewhere. Please review if this fix is heading in the right direction.

Some background info for the need to use @autoreleasepool with GCD: http://stackoverflow.com/questions/12575010/using-arc-is-it-fatal-not-to-have-an-autorelease-pool-for-every-thread.

For the correct use of SecItemCopyMatching see the GenericKeychain sample code from Apple.