Simplified weapon system and created first version of tutorial.

[viking-sudo-rm]

The interface for spellcasting was refactored so that each item implements onCast(CastEvent). The CastEvent that is passed to this function is the same one that is used as an argument for evaluating semantic nodes. This allows the second person object to be a null entity at the cast position for spell casting.

Motivated by these changes, Open was changed to be an intransitive verb that attempts to open doors at the position of the second person object. This makes it behavior more intuitive, and is also useful so that we can have a non-compositional spell in the first puzzle of the tutorial.

Finally, the old incompatible worlds were removed and a new worlds/tutorial folder was added. This tutorial has a puzzle where the player opens a door using wpi and a papyrus.

Progress on #58.
Progress on #49.
Fixes #60.