Rename 'backend' to 'publisher' schema

This reflects the naming conventions just agreed in [1].

[1]
https://gov-uk.atlassian.net/wiki/display/WH/RFC2%3A+naming+content+format+representations
Update readme to reflect naming conventions
Merge pull request #3 from alphagov/rename-backend-to-publisher

Rename 'backend' to 'publisher'