Added test checking a functor prop of ExtendedNum

ExtendedNum is a functor, so it should fulfill functor laws. Made a test
for the property that `fmap id = id` for the specific case of
ExtendedNum Integer.
Modified build-depends to include vty-ui

Beginning work on a slightly prettier text ui
Made Negamax algorithm implementation faster

findBestMove was recalculating the accumulator's score every time when
it could just store it and compare the stored valued against the score
of a new move. Now tictactoe-haskell runs in about 80% of the time it
used to take, a modest speed improvement.
Added a tentative stab at a vty UI

Gui.hs now has a short example of vty-ui with the computer playing
standard tic-tac-toe against itself.
Added border and delay

Added a short delay to the AI's moves so that I can actually see what's
going on. Also added a border around the game board to make it more
aesthetically pleasing.
Changed Makefile to make things more convenient

Removed a rule for removing a nonexistent Supergame file and added a
rule to remove the Gui executable when needed.
Cleaning up of some code in Gui

Made some edits to function names to make it more clear what is going
on.