Implement redis as cache backend
Redis  is a noSQL database with very good scaling characteristics in relation to the amount of entries and data size. This makes redis perfectly fit to our needs as a caching backend. It's a young, but very active project with currently very high development speed.
Performance measurements show that this backend easily outperforms every other existing backend in most use cases, especially when in comes to larger data sizes with tons of entries. All base operations scale O(1) with the number of cache entries (not with the number of affected entries).
There are different adapters for php, this backend implementation uses phpredis, which is a redis implementation as php module 
This backend is tested with:
- redis version >= 2.0.0-rc2, version 1.2.0 does not work
- phpredis, git version 0abb9e5ec07b8a8c20b5 from 2010-07-18, prior versions should work, but have a major performance issue with data sizes > 8kb.
This is my first real contribution to FLOW3, i hope code quality is high enough and unit tests have acceptable code coverage. Please give feedback if anything needs to be improved.
The v4 backport of this backend already exists and is in production for early adapters, it will hopefully make it to v4.5, as soon as FLOW3 accepted the implementation.