implementation of basic classes

Created a basic "game object" structure. GameObject comprise everything
that may be added to the game (interface). Block extends GameObject,
Entity extends Block. Block (abstract) contains common functionality for
basic blocks and interactions. Entity describes a further method to
allow for updating. Implemented basic class stubs for each item.
Finished ImageAsset and SpriteLoader for image processing. GameEngine
now uses a JFrame (frame) instead of relying on a GameDisplayPanel, so
that in the future full screen mode will be enabled.