Added rc, Redis cache cluster system

[fengsp]

One library that implements cache system for redis in Python. It is for use
with web applications and Python scripts. It comes with really handy apis
for easy drop-in use with common tasks, including caching decorators. It
can be used to build a cache cluster which has a routing system that allows
you to automatically set cache on different servers, sharding can be really
easy now. You can use it to batch fetch multiple cache results back, for
a cluster, it even does the job in parallel, we fetch results from all servers
concurrently, which means much higher performance.

It uses the redis server, which is a in-memory key-value data structure
server. It does not implement any other backends like filesystem and does not
intend to do so. Mostly we want to use a key-value server like redis, if you
have special needs, it is easy to write one cache decorator that stores
everything in memory using a dict or you can check out other libraries.