Improves container_tags

Changes default queryset to **get_containers_by** all_published.

Simplifies **get_containers_by_channel** query if include_children is
True.
Adds ContainerManager
Changes get_container_by_channel lookups
Removes print
Merge branch 'stable/0.2.x' of github.com:opps/opps into improvement-container_tags

Conflicts:
	docs/configuration/core.md
Merge pull request #367 from mauler/improvement-container_tags

Improvement container tags