Add scoreboard features and configuration options. Fixes gh-623, gh-45

With the new warzone configuration property "scoreboard", zone makers can
set the type of scoreboard to be used. This may be "none", "points", or
"lifepool". If the scoreboard type is not "none", then a scoreboard will
be shown on the right side of the screen in the warzone. The shown
scoreboard will display the configured option.

Also, 3-year-old feature request COMPLETED! Use the command
/zone <warzone-name> scoreboard to view a scoreboard from anywhere.