Support for App Extensions compilation + Support for keychain access group

Here is my work on making LTHPasscodeViewController working in App Extension !
There are 2 cases : 
- when using cocoapods => there is nothing special to do
- when compiling LTHPasscodeViewController in the App Extension target => define LTH_APP_EXTENSION compilation option for LTHPasscodeViewController (use prefix header ".pch" or C compilation flags or any way you find to do that).
I've also added keychain access group property "keychainAccessGroup" which is needed when you want to share keychain between the main app and its extension.