In-progress work toward a new release.

This is backwards-compatible in API terms, but also a rather large
refactor. Specifically:

* Some PEP 8 cleanup (constants mapping names/values now have
  uppercase names, with lowercase aliases for compatibility with older
  versions).

* Greatly expanded documentation.

* Tests are now using Python's unittest instead of docteat.

* Test suite massively expanded, and now includes tests which download
  and parse color definitions from standards for conformance checking,
  and which fully exercise all conversions except to/from percentage.

Additionally, a new feature:

* Experimental draft of the HTML5 parsing and serialization
  algorithms.

And cleanup/bugfixes:

* Testing discovered some color values which did not conform. These
  have been fixed.

* Support for clipping did not previously check handling of "negative
  zero" values, though it did handle them correctly. There are now
  explicit tests for these values.