Aaron

Lots of changes to Entities and accompanying tests, including..

* Constructor simplification. The parameterized constructors have been removed in favor of a SetParams() method that sets the basic positioning and size information for Entities.
* Creation of placeholders for TreeSprite, RoadSprite, and GrassSprite. These all point at default.png right now, and are ripe for being processed by the Graphics team!
* Created MockSprite constructors for all concrete Entity classes for use in unit testing. This means that *some constructors and drawing methods are not covered under World unit tests.* I don't think they should be -- you can't run them without also including the Graphics system!
* Modified the package structure for Entities. Base classes are now in bubolo.world.entity, with concrete classes (Tree, Grass, etc.) in bubolo.world.entity.concrete. Makes it a lot easier to find/add new Entity types.
* Fixed the @Author fields for bubolo.world... I think.