Clarify that empty keys/table names are not allowed.
Fix #186 by disallowing # in keys.

Also cleared up the wording of unordered key/values, as per parkr's
request.
Merge pull request #223 from BurntSushi/disallow-empty-keys

Clarify that empty keys/table names are not allowed.
Keys cannot start with [.

This change breaks backward compatibility int heory, but supporting the
syntax `[key] = 1` is pretty annoying, and TOML should be easy to parse.