SSL Pinning

Following up from #705, this pull request offers a slight revision on @OliverLetterer's excellent patch for SSL pinning. `_AFNETWORKING_PIN_SSL_CERTIFICATES_` is necessary to avoid a link error with the Security framework (the trick of `#ifdef`-ing on a header-defined macro like with `SystemConfiguration` does not work here, because `NSURLCredential` `#import`s `Security`).

This branch also documents SSL Pinning in the main header for `AFURLConnectionOperation` as well as the `SSLPinningMode` property.

@OliverLetterer @dstnbrkr, et al. — Please review these changes and test in your project. Once it's sufficiently verified, this will be merged in and tagged as part of the 1.2 release.