adding logger get/set to Config module

```ruby
config.mongoid.logger = Logger.new($stdout, :warn)
```

used to work on mongoid 2: https://github.com/mongoid/mongoid/blob/2.5.0-stable/lib/mongoid/config.rb#L146

and mongoid 3, following the docs should support the custom logger:
http://mongoid.org/en/mongoid/docs/rails.html#railties