Default Interaction map in separate class.

Refactored default interaction map into an explicit class,
which acts as a wrapper around the full CollisionInteractionMap.
A very simple player collision map.
(player, ghost) is treated the same as (ghost, player)

This is just an implementation of the CollisionMap interface
with ... instanceof.

Not-so-elegant but an easy way to circumvent Java's lack of
support for multiple dispatch for a small interaction map.
Switched to simple collision handler for now.
Merge pull request #9 from avandeursen/simple-collisions

Simple collisions