Redis Sentinel Support (HA)

[schmitch]

Actually it would be good, if the cache/session implementation would support redis sentinel.

I guess supporting it is extremly simple. it actually just needs a configuration.
basically the redis client already supports it: https://godoc.org/gopkg.in/redis.v2#example-NewFailoverClient

this will actually try to query the master and is more useful than just redis by itself

TLDR: also provide a way to use NewFailoverClient, which means that clients will "search" the master via sentinel

Cache Issue: go-macaron/cache#11