Added example world for testing.

I can't figure out a way around actually including a world in the
repository for testing purposes.  This one has multiple region files
and other important data in case I want to expand the functionality of
the existing code.
Worlds can now be read, modified, and written successfully!

Woo!  Took some serious code changes to get this far.

Sections got refactored to keep things simpler.  Regions got better
debugging.  Worlds got RegionMaps to match their ChunkMaps.
ReadRegion and friends moved to world.go temporarily.  Helper
functions were written to enable access to data based on coordinates.
And a brand new test runs through the entire process of reading the
new test world, modifying it, writing it, and reading it again.