Scoreboards

This PR continues SpaceManiac's `scoreboard` branch, updaing it for 1.8

Working so far:
* Serialization/deserialization to/from [NBT scoreboard format](http://minecraft.gamepedia.com/Scoreboard#NBT_format).
* Teams
* Player scores
* Display slots - sidebar, below name, player list

In progress:
* Team color display slots (`sidebar.team.<color>`)
* Friendly fire toggling (Glowstone doesn't have damage
* Friendly invisibles
* Objective criterion (trigger, deathCount, playerKillCount, totalKillCount, health)
* Statistic/achievement criteria