Sectors: Changed size from 8xINFx8 -> 8x8x8
This means you can build as high or low as you like, vertical travel is
equivalent to horizontal travel performance wise.

Also optimized world.change_sectors
Saving: Added Region Save Mode
A very scalable system. Sectors (8x8x8 blocks) are stored 4x4x4 in headerless
files, 2 bytes per block. Position data is not stored, but inferred based on
the byte offset within the file.

Whole regions (32x32x32 blocks) are read at once when world._show_sector asks.
Sectors can be written to individually, though without a dirty_sectors system,
it currently writes all sectors in memory to disk during a save.
Merge pull request #60 from Nebual/saving-region

Saving: Added Region Save Mode