Updated terrain generation

-Now generates land away from the sides; top, bottom, left and right are
now all water, just like in Civ5
-Fixed a bug in the diamond square method where it wasnt taking the
value below it, not too big of an impact
-Allowed diamond square to generate over previously generated tiles,
improves the fractal shape
-Added Fractional Brownian Motion to the terrain generation, using
diamond square to generate the noise instead of perlin/simplex;
generates better fractals