Update to Minecraft 1.8, update version to 2.2.6

More changes were needed than usual.

- Updated package versions.
- Updated for new NMS method names.
- Bit of troubles with dependencies. See POM file for an explanation.
- Updated to use methods of Guava 17.
- Spigot-API has less deprecated methods than Bukkit, so some
@SuppressWarnings("deprecation") are no longer necessary.