Log cache access

Adds "Cache MISS, Cache HIT, set cache value, delete cache value" etc.
information to the log file.

Note: entries are logged at the LOG_INFO level.

Installation
============
add "addPlugin('CacheLog');"
to the bottom of your config.php

Settings
========
none

Example
=======
Note that since most caching plugins return false for StartCache*
methods, you should add this plugin before them, i.e.

    addPlugin('CacheLog');
    addPlugin('XCache');