Add a data model

[narrowtux]

Right now, the model is the database, which is fine, although the code will look way nicer when using actual model classes in Java, which support operations on them (like set the challenge state to UNDONE), and those operations would automatically inflict the appropriate changes across the data model (deduction of points, lowering the rank, and so on).

The advantage would be a testable data model, to guarantee that the business-logic behind challenges always works, and an easier way to deal with data in general (using Java Objects with nice getters and type safety instead of writing 50 SQL Queries manually).

[narrowtux]

This will also make #78 easier since it can just give us the actual Bukkit Player object from the UUID in the database and we don't have to make a table for the player's name.

[narrowtux]

See https://github.com/CoreNetwork/Challenges/tree/pojomodel