Convert gcylc stopped state to str in case it is None.
Enhanced the broadcast command.

1) Provided an interface to the broadcast expire method.
2) Allowed clearing broadcasts by cycle time or namespace.
3) Rewrote the command help.
Typo corrected in broadcast command help.
Made limitations into features.
Rewrote broadcast command help.
Minor broadcast command help change.
Command help clarification, again.
More broadcast command help tidying.
Remove just-deprecasted command option.
Be explicit about broadcast defaults.
Fix casting of datetime in scheduler.
Merge pull request #631 from arjclark/fix-shutdown-datetime

cylc shutdown|stop: fix using a datetime for shutdown
Merge pull request #626 from hjoliver/gcylc.str-stopped-state

Convert gcylc stopped state to str in case it is None.
Updated pyro URL in the user guide.
Updated the gh-pages index page template.

Pyro URL update, and other changes pulled back from the live index page.
Merge pull request #628 from hjoliver/broadcast-enhancement

Enhanced the broadcast command.
Put command-specific option help first.
Minor hot fix: completes 3e538a
Merge pull request #632 from hjoliver/pyro-url-update

Pyro url update
Merge pull request #633 from hjoliver/reorder-command-options

Put command-specific option help first.