Db and default values in Redis(Native)Client constructor

I think it should be possible to specify which database to use in the constructor. As it is now, setting .Db is not very discoverable (IMO). I've added default values to password and db to avoid constructor explosion.

I'm also planning on adding a constructor that accepts a connection string (like `"Server=localhost;Password=foo;Db=5"` which makes it easier to configure connections through a configuration file. What do you think about that?