ItemType, WeaponType

basic classes and test prefabs
Inventory: Pickup/Drop/Fire Weapons

Lots of stuff that turned out quite ugly.
But now we can pick up items, drop them again, and even fire some
bullets!
ItemType now has configurable pickup graphics

Small changes to code to allow for an ItemType to specify the prefab to
be shown for a pickup. Pickups create this prefab in the Start method
and adds it as child of itself.
Projectile using Physics.RayCast

Projectiles now use raycasts to detect hits.
Also added some more configurable parameters to WeaponType.
WeaponPickupSpawn : PickupSpawn

Now we can drag a WeaponPickupSpawn into the world, set some parameters
(spawn times, weapon type and ammo) and it automatically spawns
WeaponPickups.
Also, Pickups can now spin!