Support for client-side manual trust evaluation (cert pinning) via delegate methods

As per [Apple Technical Note on Server Trust Evaluation](http://developer.apple.com/library/ios/#technotes/tn2232/_index.html), partly based on work by @dirkx on [client certificates in CocoaHTTPServer](https://github.com/robbiehanson/CocoaHTTPServer/pull/26) I added support for manual trust evaluation in client SSL sockets. 

This also lets us use SecureTransport with invalid/self-signed certificates, for which right now `kCFStreamSSLValidatesCertificateChain` was needed (which would make socket fall back to CF for SSL).