Multiple updates

- Added generators.FractalWorldGenerator which uses a fractal algorithm
  to generate terrain with variable size, roughness and deviation.

- Added img_terrain.ImageWorldGenerator which takes an image as input
  and generates terrain based on the average color value of each pixel.

- Made sector size smaller and view distance larger. Default world
  generator is now the FractalWorldGenerator.
Merge branch 'master' of https://github.com/tfaris/Minecraft into imaged_terrain