Added gfx::Rect and gfx::Sprite classes
AssetManager now has sole ownership of assets

Instead of returning shared_ptrs, returns plain pointers, and holds
requested objects in unique_ptrs (until the AssetManager is destroyed)
Created header files for all primary game logic

Features classes (Feature, City, Cloister, Road, Farm) need to be
specified still.

Follower spec is incomplete.  Player spec might need a few more
functions, but is mostly complete.  Scenario needs more functions.  Pile
is complete (though Tile isn't).  Board is almost complete - needs
unique_ptr for tile currently being placed.