Added a script to compare current parser output to cache

* This works on a random set of pages in a namespace and can
  be used to compare different PHP interpreters.
* This can, like any maintenance script, be used with --profiler.

Also includes I8905ae02b: Tweaks to compareParserCache.php

* Skip parsing if there is no cache entry to compare to
* Mention the total number of pages with differences

Change-Id: Ica69a3ef27df29af1c6e4dc4c8413b55a03df49e
(cherry picked from commit 380b075756a15f82a158be5c3a4283188208c820)