[WIP] Scoreboard API

This PR adds a Scoreboard API, supporting all of the (newer) scoreboard features like statistic criteria and colored sidebar slots.

**TODO**:
- [ ] Implement colored sidebar slots
- [ ] Implement statistics criteria (waiting on https://github.com/SpongePowered/SpongeAPI/pull/220)

The relationship between Objectives and Scoreboards still needs to be decided.

From IRC:
```
<gratimax> however, possibly a better way to do this would be each scoreboard is a view of objectives, so multiple scoreboards can view the same objective
<gratimax> then the data that a player has with regards to a certain objective is stored by the objective, conceptually, not the scoreboard
<gratimax> this contradicts what objectives look like internally(saved per-scoreboard), but considering that per-player scoreboards are hack I think this is a good direction to go
```
we need documentation here about the exception