Introduce typed DTOs in API arguments and responses

## Long story short

Currently collections have different return values, containers.list() returns list of container objects whereas all other objects return the raw response from Docker engine as dict.

* Expected behaviour:
All object.list() calls return object collections
* Actual behaviour:
Return values are different for container and all others objects (volume, image, ...)

## Details

As a user I'd expect to get a collection of objects from a .list() call. However, it might make sense to have access to the raw response too.

docker-py has different classes for each object, one api object for low-level stuff and a model object for high-level access. This seems to be a good approach in my opinion.

I'd be willing to provide a PR, but first I'd like to get some input on where to go, maybe there are some things I didn't consider,.