Change Document.submitter from a string to a json / dict / mapping

MANUAL-UPGRADE: Change column type of document submitter to json in
postgres db
Create BaseContent as a base class for Document and Binder
Add authors field in metadata and set it to the initial submitter

MANUAL-UPGRADE: Add authors field in document in postgres db
Merge pull request #66 from Connexions/submitter-as-json

Submitter as json