Add support for iOS 8 Keychain API to detect passcode presence.

This resolves issue: https://github.com/project-imas/passcode-check/issues/4

I added support for the iOS 8 Keychain API based on snippet here: http://pastebin.com/T9YwEjnL

Additionally, there is handling for the API being missing, in which case the class method returns false (even though a passcode might be set) due to the constant not being available.