Added monitor headers information to 'Montior' section

Motivation:
Currently, the 'register' section contains the following:
"The push server SHOULD also provide the "channel" link and expiration
information in response to requests to the "monitor" resource."

The above is not mentioned in the 'monitor' section which might cause it
to be missed.
Merge pull request #18 from danbev/monitor-response-headers

Added monitor headers information to 'Montior' section
Reworking intro and overview
Lots and lots of changes
Lots more edits
Adding IANA considerations
Addressing reliability concerns.Closes #9.
Updating bearer token language; closes #8