Redis Cache Handler

Add Redis cache support using the Predis Redis client library.

Supports TCP/UNIX/HTTP connections to Redis (i.e. same as the Predis library). Note, if HTTP is required then installation of this tool is required: https://github.com/nicolasff/webdis

Here is a sample configuration array covering the main options of how Redis caching can be configured:

```
$config_options = array(
  'cache_handler' => 'cache.xPDORedis',
  'redis_server' => 'unix:///var/run/redis/redis.sock?database=0',
  'system_settings_redis_server' => 'unix:///var/run/redis/redis.sock?database=1',
  'lexicon_topics_redis_server' => '127.0.0.1:6380?database=0',
  'resource_redis_server' => '127.0.0.1:6380?database=1'
);
```

It should be noted in the above configuration array, that UNIX sockets are only for when Redis is on the same server as MODX. If remote server node access is required to Redis, then TCP connections should be used.