Adds SSLPinningMode to AFURLConnectionOperation and defaultSSLPinMode to AFHTTPClient.
Fixes memory leak.
Gets rid of for loop to check if public key is present.
Stashing merge
Replacing #ifdef macro check for Security to _AFNETWORKING_PIN_SSL_CERTIFICATES_
Removing #warning about Security framework
Fixing compiler warnings in SSL pinning initializers

Minor refactoring
Adding Security framework to example projects and adding #define for _AFNETWORKING_PIN_SSL_CERTIFICATES_
Adding documentation about SSL Pinning
Minor reformatting of AFHTTPClient constants documentation
Adding Security frameworks to podspec, and defining _AFNETWORKING_PIN_SSL_CERTIFICATES_
Adds `AFSSLPinningModeNone`.
Minor formatting and documentation edits
Merge branch 'experimental-ssl-pinning'