Sentinel support

Was #452.
If `get-master-addr-by-name` returns `nil`, the above `sentinel_detect` will happily iterate all sentinels and then return the array (as it is the value of the each call). This will lead to some surprising error messages:

```
/home/badboy/code/redis-rb/lib/redis/connection/ruby.rb:208:in `[]': no implicit conversion of Symbol into Integer (TypeError)
```

We should catch this