add password support for redis connects in cache and session

This code was originally taken from; https://github.com/durana/moodle-cachestore_redis
Special thanks to Mark Nielsen and Adam Durana for their work on this.

Since the original import;
- Mode session support has been added.
- Some cleanup of names has been made.
- Data guarantee and references support was added.

Damyon Wiese authored May 03, 2012 and Dan Poltawski committed May 03, 2012

* Includes an assignment upgrade tool to convert from the old mod_assignment.
* Hides mod_assignment on new installs

Most significantly, all behaviours now work.

MDL-23068 removing obsolete $result and removing empty upgrade files that would be probably never used at all + minor cleanup/fixing

MDL-20700 coding style cleanup - cvs keywords removed, closign php tag removed, trailing whitespace cleanup

MDL-16438 fixed incomplete plugin centralisation, sorry; unfortunately install is kind of non-working now; this may break current dev installs :-(

MDL-17188 "effective question weight equation cannot cope with negative covariance of question grades" excluding such questions from equation

MDL-14202 "Replace Item Analysis Report with new improved 'Statistics' report." finished statistics report. This patch includes some changes to lib/tablelib.php so that it is possible to export the content of a table as part of a multi table export - with mutliple tables / multiple worksheets.

MDL-16002 rewritten upgrade locking and better upgrade progress tracking; MDL-16070 Do not use $a[0] syntax in lang packs + various other upgrade improvements and fixes

MDL-7308 - Check all the DB columns used to store grades, and make sure they all use a consistent type.

MDL-14208 Cache results of calculations, "recompute now" functionality -- forgot one field for questions stats.