Replace doublecircle with tripleoctagon for non-trigger families.

Closes #557.
Allow family names in all special task categories.

Shorthand for listing all member tasks.
Updated documentation.
Merge pull request #566 from hjoliver/557.tripleoctagon

Replace doublecircle with tripleoctagon for non-trigger families.
Merge pull request #567 from hjoliver/563.special-task-families

Allow family names in all special task categories.
Merge branch 'master' into gui-collapsed-families

Conflicts:
	lib/cylc/config.py
Replace another doublecircle with tripleoctagon.