Added basic list of achievements

Added a fairly basic idea of how achievements can work. Supports 1.8 achievements.
Open to input.
This should be an interface so it can be implemented in Sponge. Take a look at [`BlockType`](https://github.com/SpongePowered/SpongeAPI/blob/master/src/main/java/org/spongepowered/api/block/BlockType.java) and [`BlockTypes`](https://github.com/SpongePowered/SpongeAPI/blob/master/src/main/java/org/spongepowered/api/block/BlockTypes.java) for an example.