Added me as author of cave and collision tilesets

Also attributed cave.png to Modanung and Georg since as far as I
remember they also did parts of that tileset (like the water and the
carpet). I only drew the walls and arches.
Added V0id for weapon-bansheebow.png and arrowh-banshee.png
Merge pull request #15 from bjorn/master

Some license.txt updates