First set of updates for recent discussions

Changes:
- Added optional registerOptions parameter in register interface.
- Added optional body in push message
- Remove System Messages: changed references to "System Messages" to
"events" generically. "System Messages" section was renamed "Push
Events".
- Globally changed "notitificaton" to "message".
- Added "7.3 Push Registration Persistence" to clarify how registrations
preserved between sessions.
- Added "5.1 Push Server Discovery and API Use" to illustrate how
webapps may (hypothetically) determine the applicable push system, and
use Push API data in Push Server API requests.
- Edited "5. Push Framework" for clarity, to clarify what information is
passing through push service, and how user-agents are authenticated.
- Added "5.1 Push Server Discovery and API Use" to illustrate how
webapps may (hypothetically) determine the applicable push system, and
use Push API data in Push Server API requests.
- Updated the example code based upon Alex's service worker based
example. It needs to be enhanced to include other push systems as
examples, e.g. include their data in the registerOptions.
Merge pull request #22 from blsaws/develop

First set of updates for recent discussions
Second set of updates for recent discussions, TAG comments, etc

Changes:
-	Added "7.4 Push Registration Uniqueness" to clarify for TAG comment
that same-origin webpages do not share registrations.
-	Added text to "7.3 Push Registration Persistence" to clarify for TAG
comment that how user-agents invoke inactive webapp is not described
here... expected in Service Workers
-	Added text to "3. Terminology" and "4. Security and privacy
considerations" for TAG comment, to clarify what is meant by "express
permission"
- In "7.2 Steps", per TAG comment removed the text "or alternatively
that the requesting webapp has permission to deactivate such push
registration", focusing the NoModificationAllowedError purely on the
validity of the passed pushRegistrationId for this webapp.
-	Added notes for consideration of various TAG comments and suggestions.
- Added text to "7.4 Push Registration Uniqueness" for TAG comment, to
clarify that webapp association of specific registrations to "message
streams or sources" is an application-layer concern, and that webapp
servers can provide means to map registrations to specific webapp
features/functions as needed.
-	Fixed various missing anchors to code elements
-	Fixed respec issues
-	Fixed normative vs informative reference issues
Merge pull request #23 from blsaws/develop

Second set of updates for recent discussions, TAG comments, etc