Added README.md for world package.

This README will help me develop and maintain the world package,
hopefully.
Reworked NBT code to be more idiomatic.

Added accessors, refactored Tags map into separate maps, progress!
Added initial debug support for NBT package.

Set nbt.Debug to true and some debug output is printed.
Added initial file support to NBT package.

First pass at file support reads compressed and uncompressed files.
Write support comes next.
Added test files for new file code.

Looks like the test files are back!
Added tests for file methods.

The new methods are now tested.
Removed file tests from NBT tests.

Previously two file tests were included in the NBT tests.  Now that
the file code has been broken out into a separate file, so have those
tests.
Tags can now be read from and written to files!

Both compressed and uncompressed files are supported.
Merge branch 'file-support' into develop
Added README.md for world package.

This README will help me develop and maintain the world package,
hopefully.
Merge branch 'world-package' of github.com:mathuin/terroir into world-package
First pass at level.dat creation.

The level.dat works in the game and passes the tests.
Updated docs and code to use new level features.

The documentation now contains references to level files.  The world
package now uses the level file generator to build very spare worlds
that actually work in the game.  All tests pass, coverage is better
than I expected.
Refactored level-writing code.

Subject says it all!  Still passing tests.
First pass at chunks and regions.

Used README.md information to create structs for chunks and regions.
Removed superfluous comment from world.go.
More work on the chunk code, including some tests!

Coverage percentage is dropping.  First rough chunk tag writing done.
New NBT utility functions aid in building worlds.

Lots of compound tag construction, so I refactored the code and put it
in the NBT package.
Refactored chunk and level code.

Broke down Half and Double functions into simpler more testable components.

Used new NBT functions in chunk and section construction.

Made a note to remind myself to fix the level tests to use maps.
Moved utility funcs and tests, and more.

Also refactored level tests to use requiredTags approach and made
progress towards testing chunks and sections.
Changed chunk and utils functions to be more generic.

Also added more accessors for chunks and sections.  Still better than
half code coverage with testing, but should improve soon.