Broadcast Plugin Support

Instead of looping through all users, offload the broadcast to Openfire via the broadcast plugin.

Should greatly improve broadcast speeds.

Addresses #104 (closed)

This does however introduce some security issues: mainly that clever users would be able to figure out how to broadcast their own groups. As per the plugin's readme, we could get users to set:

plugin.broadcast.disableGroupPermissions    true
plugin.broadcast.groupMembersAllowed        false
plugin.broadcast.allowedUsers               broadcast@yourdomain.com

The broadcast user would need to be an administrator which raises its own security problems, but it's a step in the right direction.