Obtaining a manifest should be before processing #234.
Consolidate the following under 'Associating a resource with a manifest':
* Linking to a manifest
* The "manifest.json" well-known URI
(IANA considerations now only has the bits required for registration)
Split the Manifest section into two top-level sections:
* Manifest processing model
* Manifest members
Add a brief intro to the Manifest processing model.
Move Extensibility into Conformance.
Clarify proprietary extensions refer to new members.
Merge pull request #239 from anssiko/reorg

Improve the organization of the spec, non-normative changes