Code cleanup

- Added Delegates.cs static class to contain re-useable delegates, such
as QuitFunction
- Removed unnecessary caching of ship textures in GameScreen.cs - XNA
Content Manager already does this - once an asset is loaded, there is no
overhead to call Content.Load<T>(assetName).

NOTE: There may be a benefit to statically pre-loading all assets at the
start of the game into a class structure that can be used throughout the
game to pull individual Textures as needed. The benefit would be code
readability, single place to manage content loading process, and
guarantee that all assets are pre-loaded, thereby saving disk access
time during game play.
Added GameContent loader class; re-org'd content

- Improved organization for Game Content by sub-folders.
- Loading of content is now done in a singleton class GameContent, which
ensures pre-loading of game assets, and provides structured approach to
get to content in code via class hierarchy, enumerators, and indexers.
- General code cleanup