Created CRedisCache (backport from yii2)

redis cache uses http://redis.io/ as storage backend.
Feature was requested in discussion in #1302
code style and documentation for CRedisCache
added redis cache to definitive guide
redis drop millisecond support to not require latest redis verion

yii2 will have this feature.
enabled redis on travis-ci
Merge pull request #2436 from cebe/redis-cache

Created CRedisCache (backport from yii2)