RedisOrm.redis to set proper redis instance/DB

hi there,

first of all thanks for the gem :)
I found a bit odd thing, you were using $redis instance to work with, but I already got that full of data of other kind and I actually use 10 out of 16 redis DBs available to us... and one of them has to be $redis already and I don't want to mix these two together...

Therefore what I did here I gave myself redis variable to module RedisOrm, so I can freely set it for example to this:
```RedisOrm.redis = Redis.new(db:'9')``` and BANG I got it separated and I am satisfied now :)

I already added info into readme, and also tests are still passing, I hope you merge and upload to rubygems, for now I am stuck with my github, looking forward to merge :)

have a nice day :sun_with_face: