opus

Updates from mailing list and other small fixes.

* Bump the document date.
* Mandate that the ID header must complete on the first page (to
   remove any ambiguities about this requirement in RFC 3533).
* Remove reundant wording that rillian forgot to remove in 360a4117.
* Split the "Granule Position" section into subsections.
* Move the first paragraph of the "Other Implementation Notes"
   section into the "Granule Position" section, add general seeking
   implementation guidance, and be specific about the interaction
   between pre-roll and pre-skip.
* Retitle the remaining contents of the "Other Implementation Notes"
   section to "Packet Size Limits"
* Specify that all the header fields are REQUIRED (and add a
   description of the Channel Mapping Table as a whole, so we can
   say when it is REQUIRED).
* Specify that implementations MUST NOT reject headers with extra
   data if they have an unknown minor version number.
* Add a reference to RFC 3629 (UTF-8).
* Minor formatting adjustments to vorbis-trim and vorbis-mapping
   cites.
* Eliminate semicolons and terrible "Else, if" constructs.