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.
Merge pull request #628 from hjoliver/broadcast-enhancement

Enhanced the broadcast command.