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