SSL pinning revised

* As discussed in #877, AFNetworking will now trust each pinned certificate and its derived certificates (a91f40bae5926421809bb52cc9a1e2872bd3d2ef).
* I think all operations enqueued to an `AFHTTPClient` should inherit the default pinning mode if not explicitly set otherwise. Thats what the method swizzling in 05a991bdaf63ca5e5dbbc627d92cac6936f1dfbe is for.