[contacts_api] IAddressBook moved to OCP as it's used by apps to provide access to their contact data

lib/public/contacts.php

 		}
 
 		/**
-		 * @param \OC\IAddressBook $address_book
+		 * @param \OCP\IAddressBook $address_book
 		 */
-		public static function registerAddressBook(\OC\IAddressBook $address_book) {
+		public static function registerAddressBook(\OCP\IAddressBook $address_book) {
 			self::$address_books[$address_book->getKey()] = $address_book;
 		}
 
 		/**
-		 * @param \OC\IAddressBook $address_book
+		 * @param \OCP\IAddressBook $address_book
 		 */
-		public static function unregisterAddressBook(\OC\IAddressBook $address_book) {
+		public static function unregisterAddressBook(\OCP\IAddressBook $address_book) {
 			unset(self::$address_books[$address_book->getKey()]);
 		}
 
 		}
 
 		/**
-		 * @var \OC\IAddressBook[] which holds all registered address books
+		 * @var \OCP\IAddressBook[] which holds all registered address books
 		 */
 		private static $address_books = array();
 	}

lib/iaddressbook.php → lib/public/iaddressbook.php

  *
  */
 
-namespace OC {
+// use OCP namespace for all classes that are considered public.
+// This means that they should be used by apps instead of the internal ownCloud classes
+namespace OCP {
 	interface IAddressBook {
 
 		/**