[Resolver] Fix scoping of subspec dependencies by maintaining from_subspec_dependency? attribute
[ResolverSpec] Add spec covering the exclusion of subspec dependencies of incompatible platforms when there's only one target platform

Done by using AFNetworking 2.4.1 and its UIKit subspec (iOS only) on an OS X platform
[Resolver] Rename matching_dependencies_from_node lamdba to be more readable