Completed initial unit implementation

-Enumerations for unit types.
-Two different constructors for creeps/buildings
-Added a few potential function definitions I think we'll need.
-Completed building functionality:
Press 1-7 to choose building type (only normal (1) works right now
because it's the only one with an image).
Press b to begin choosing an area to build on.
Click the mouse to confirm location and build building.