Refactoring of spec

Split definitions and processing rules.
Change definition of reference set (issue #189).
Update management of reference set (issue #214).
Add example of 32 bytes entry structure (issue #191).
Change name to HPACK.
Modify definition of encoding concepts.
Add some cross-references.
Add final emission (part of issue #214).
Remove old sections.
Add description of header set.

Move definition of header block.
Improve reference set emission.
Add multiple header guidance (issue #198).
Modify initial header tables (issue #188).
Fixes issue #204.
Improve presentation of Header Representations.
Editions.