Allow Backup to sync remote redis server.

Use `redis-cli --rdb filename` to sync remote redis server and write data to local.

We can then use it like following

```ruby
database Redis do |db|
    # sync_remote! will dump remote data to the following path/name
    db.name             = "redis_db_name"
    db.path               = "/usr/local/var/db/redis"
    db.password       = "password"

    # If there is no specified host, --rdb will dump local redis-server
    db.host = 33.33.33.10
    db.port = 6379
    db.sync_remote = true
  end
```

Notice that if there is no specified host, `redis-cli --rdb` will dump local redis-server.
In other words, it can somehow replace the function of `redis-cli SAVE`. But i'm not sure whether should we replace `invoke_save!` by `sync_remote!`.

If these codes are OK, I will update README or wiki to add api document later.

This doesn't include a test because I couldn't quite figure out the test suite, but I want to start the conversation.

If there is any concern, please let me know. Thanks!