Redis cache backend

This pull request adds [Redis](http://redis.io) as an optional cache backend.

**Explanation:**

The organization I work for ([ParkMe](http://parkme.com)) currently uses the mapserver with the memcached backend to render parking location information. Recent updates to our infrastructure have led to a roadmap in which we phase out memcached in favor of [redis](http://redis.io). This is a provisional take at implementing a mapserver cache backend for redis that is production-ready.
```mapcache_cache cache;``` MUST be the first member of the ```mapcache_cache_redis``` struct as that's how structure inheritance is performed. I'm actually quite surprised that you got anything working without this...?