Don't link all groups to all group

Although all children are technically a child of all, it simplifies
the graph if we only link groups without other parents to all.
Fixed documentation and graphs to reflect recent changes
Memoisation of parent group graphing
Merge branch 'dont_link_all_groups_to_all' into group_graphs

Conflicts:
	README.md
	bin/ansible-inventory-grapher
	test/test-prod-web-server-78a.png