implementation of Redis caching

In PR https://github.com/joomla/joomla-cms/pull/3397 user @andrewnester created the posibility for Redis Caching in Joomla. But it wasn't complete and there where some obsolete files included in the PR.
There for I've created a new pull request.

- apply patch
- go to Global Configuration >> System
- notice cache handler "Redis" in the dropdown
- select cache handler "Redis" and new config fields will appear
- if needed apply config. (host, port and auth can vary at some hosting providers)
- save configuration
- refresh your website while keep an eye on the Redis monitor
- if everything works well, you'll see that Redis caching will serve the website.

Credits to @andrewnester for the initial pull request. (forked from memcached)