Started work on sticky bomb
Misc. changes to bombs and world

Bombs now have explosion logic in bomb:explode().
world:move_entity() now checks if the object to be moved has a radius,
and thus whether it should be in the spatial hash.
Added OnDeath callback; sticky bombs use Health
Bombs explode when Health=0; cleaned bomb code
Made sticky bombs stick to outside of target
Made test weapon var names in test.lua more general
Weapon firing requires Rotation component
Death explosions made optional
Renamed some weapon/projectile arguments
Updated weapon TODO

Removed rocket launcher (which is basically the bomb launcher) and
sticky bomb launcher; added stuff about menus I have to make when I get
to it.