XP Tome

- Added a new XP Tome item with a new internal way of handling experience storage. This means, that you have to convert your old XP Tomes by simply rightclicking them. You will not lose any experience. The old item is kept to stay compatible with previous XP Tome releases.
- Added configuration option to set the maximum experience the XP Tome can store.
- Added configuration option to set a percentage of experience that will get lost when retrieving stored experience from the XP Tome.
- Added configuration option to only retrieve a maximum of one level at a time.
- Added configuration option to only store a maximum of one level at a time.

The configuration file can be found in the serverconfig folder of your world.