Image now just contains a filename.  and params now all need to be Strings (to make the md representation easy)
Merge branch 'develop' of https://github.com/Sage-Bionetworks/Synapse-Repository-Services into develop
Merge pull request #267 from jay-hodgson/develop

SWC-357
Merge pull request #268 from eric-wu/develop

Add a worker to sync RDS and DynamoDb periodically and other trivial improvements