Move the Log class to system/core/

It is a core class after all, I guess somebody forgot it when the rest of them were moved.
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index 77af733..f3509d0 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -46,7 +46,8 @@
       Only entries in ``$autoload['libraries']`` are auto-loaded now.
    -  Removed previously deprecated EXT constant.
    -  Updated all classes to be written in PHP 5 style, with visibility declarations and no ``var`` usage for properties.
-   -  Moved error templates to *application/views/errors*.
+   -  Moved error templates to *application/views/errors/*.
+   -  Moved the Log class to *application/core/*
    -  Global config files are loaded first, then environment ones. Environment config keys overwrite base ones, allowing to only set the keys we want changed per environment.
    -  Changed detection of ``$view_folder`` so that if it's not found in the current path, it will now also be searched for under the application folder.
    -  Path constants BASEPATH, APPPATH and VIEWPATH are now (internally) defined as absolute paths.