SSL pinning tests

I added a few SSL pinning tests. There are a few things to note:

* certificate and public key pinning is finally working on ios and os x (by falling back to SecItemExport on os x, #1028)
* i tried to use the new AFTestURLProtocol class to send the authentication challenges which turned out to be kind of impossible for me. NSURLProtectionSpace is relying internally on the private CFURLProtectionSpace and I was unable to send a proper authentication including the correct serverIdentity. Therefore I decided to call `[operation connection:nil willSendRequestForAuthenticationChallenge:authenticationChallenge];` manually.