Replace is_file() with the faster file_exists()
(where it makes sense)

Also:
 - Implemented caching of configuration arrays for smileys, foreign characters and doctypes.
 - Implemented cascading-style loading of configuration files (except for library configs, DB and constants.php).
diff --git a/system/core/Hooks.php b/system/core/Hooks.php
index 59759e0..17f6a02 100644
--- a/system/core/Hooks.php
+++ b/system/core/Hooks.php
@@ -81,11 +81,12 @@
 		}
 
 		// Grab the "hooks" definition file.
-		if (is_file(APPPATH.'config/'.ENVIRONMENT.'/hooks.php'))
+		if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/hooks.php'))
 		{
 			include(APPPATH.'config/'.ENVIRONMENT.'/hooks.php');
 		}
-		elseif (is_file(APPPATH.'config/hooks.php'))
+
+		if (file_exists(APPPATH.'config/hooks.php'))
 		{
 			include(APPPATH.'config/hooks.php');
 		}