Merge pull request #4 from Computerology/taintedwind

Reverting some changes to GameTick, and removing unused imports
Entity performance test, moved GameTick

- removing all entities from world brought FPS back to where it should be (which confirms that the chunk handling is not the bottleneck, it is purely entity code)
- also moved GameTick inside the World class
- methods in Grid do not create  instances of Rectangle to determine point intersection