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.