Updates binary encoding to handle extensions and content-type per spec.

Update marshaller to handle both structured and binary formats. Previously, it would try structured first, even if the content-type did not match, and then ignore all the headers, producing very wimpy events with no data.

• Link to issue this resolves
  I was too lazy to file an issue while debugging.

• What I did
  I wrote a small python program using Flask to log cloud events, and to report them in a webpage. I'll be putting in in a pull request to https://github.com/knative/docs soon.

• How I did it
  I used the following command to POST cloudevents to the flask program I wrote above.

curl -v -d '{}' \
  -H 'CE-SpecVersion: 0.2' \
  -H 'Content-Type: application/json; charset=utf-8' \
  -H 'CE-Source: "http://manual/"' \
  -H 'CE-Type: "local.badidea"' \
  -H 'CE-Id: 123' \
  -H 'CE-Time: "2019-01-11T23:40:00Z"' \
  -H 'CE-test: foo' \
  -H 'CE-test: bar' \
  -H 'CE-sampling: 2' \
  -H 'traceparent: aauau' \
  http://127.0.0.1:5000/

• How to verify it
  I updated the tests.

• One line description for the changelog

* Updated library to better support binary encoding