Added subnet tagging.
Added documentation for subnets: tags: option
Refactored subnet tagging to account for AWS delays; added 'tags' attribute to 'subnet' node in the returned json.
Renamed subnet's 'tags' attribute into 'instance_tags' to distinguish it from Ansible's own 'tags' and to conform to ec2 module naming for AWS tags.
Renamed instance_tags to resource_tags based on community feedback.