Replace references to web intents with "push service extension"

Since web intents is shelved for now, we need a fallback method for
user-managed push service selection/configuration. Browser extensions
are a reasonable method, where supported.
Complete example code

The example code should show at least one usage of each aspect of the
API.
if none, registrations() returns 0 length array

Clarify in "7.2 Steps" that if pushRegistrations is length 0 that
indicates there are no registrations
Push server *may* skip version notifications

The current text assumes that push servers will only send the latest
notification version, but this is an unnecessary assumption.
Merge pull request #19 from blsaws/develop

Edits for new draft