Clarify whitespace and newline rules.

This PR greatly clarifies the use of whitespace and newlines in TOML. A few notes and ramifications:

* Behavior on Windows is noted.
* Key and table names cannot contain any whitespace, even around brackets or dots.
* Keys and dot-separated table parts follow the exact same rules.
Indenting is covered by the previous statement that "Whitespace is ignored around key names and values.", so I thought it to be redundant. Also, the ability to indent is a weird way to segue into nested tables, and makes it sound as if indentation might carry some semantic value.