Refactored gameboard functions

Added a simple test function that can be called with the "--test" flag.
Made the gameboard object (T_GB) an immutable type. All functions modifying it now
return a new copy of the original with the changes applied.

 On branch dev_ben
 Changes to be committed:

	modified:   ../README.md
	modified:   ../src/gb.c
	modified:   ../src/gb.h
	modified:   ../src/main.c