[Resolver] Add filtering for subspecs included via subspec_dependencies
[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
[ResolverSpec] Clarify spec description
[Resolver] Extract valid_dependencies_for_target_from_node method
[Resolver] Extract edge_is_valid_for_target? method
Merge pull request #2760 from CocoaPods/seg-from-subspec-dependencies

[WIP] [Resolver] Add filtering for subspecs included via subspec_dependencies