Add HTTP request logger

@sosedoff

Does this do kind of what you're looking for?

```
[1] pry(main)> require 'docker'
=> true
[2] pry(main)> require 'logger'
=> true
[3] pry(main)> Docker.logger = Logger.new(STDOUT); nil
=> nil
[4] pry(main)> Docker::Image.all
I, [2013-12-20T12:18:24.776713 #20286]  INFO -- : [:get, "/v1.6/images/json", {}, nil]
=> # Image stuff
```