Merge pull request #9 from jcao219/master

try to fix the hexagon layout
Implemented left-click and right-click options

-cells in the world now react to left and right clicks on them
-added some JavaDoc for a majority of the methods
-updated the World.java class to allow for removing objects at a given
location
-also changed the printCritter() method to return a String instead of
printing out the Critter
Implement time step