Add simple scoreboard API

[PetteriM1]

Implemented a simple API to create scoreboards. Managing the scoreboards is fully left to plugin developers.

Usage:

• Create a new scoreboard new Scoreboard(String title, SortOrder, DisplaySlot).
• Show it to player scoreboard.showTo(Player).
• Add/modify scores scoreboard.setScore(String scorer, int score).
• Remove scores scoreboard.removeScore(String scorer) or scoreboard.clear().
• And hide it at the latest when player quits scoreboard.hideFor(Player).
• Non duplicate modifications to scores send updates to viewers automatically. This can be paused temporarily for efficient bulk modifications using holdUpdates() and unholdUpdates().