Add Redis support as a caching backend

It would be fairly easy to add support for Redis as a cache backend. I've been using Redis with CiviCRM 4.4 and 4.6 for some time now on Pantheon. I've formatted a patch which I hope is acceptable for 4.7.

(I know about this issue: https://issues.civicrm.org/jira/browse/CRM-14457 but I wanted to contribute this as a patch and get it into CiviCRM core. I'm not convinced adding this as an extension is worth the effort when the Redis class can be so similar to memcache. It's not like there are a lot of other cache backends out there in popular use.)