Add support for filtering images and containers

[`filters` is a json encoded value of the filters (a map[string][string]) to process on the images list. ][1]
The tricky thing is that we must convert boolean value to string and any filter value to list to make a `map[string][string]` json format

[1]: https://docs.docker.com/reference/api/docker_remote_api_v1.14/#list-images
Merge branch 'filters' of github.com:irachex/docker-py into irachex-filters

Conflicts:
	docker/utils/__init__.py
	docker/utils/utils.py
	tests/utils_test.py
Updated tox.ini to also run utils_test.py tests
Merge branch 'irachex-filters'