Description of problem:
During the export of a manifest the adapters need an API hook to add files to the signed export.

Per the design at: https://trello.com/c/oVy11kLi we need:
1) Add an optional version # argument to the export REST API.
2) Create an adapter API that takes 3 arguments. 
  * The directory on the filesystem where any new files should be added
  * The consumer object for whom the export is being created. 
  * The version number passed into the api (1). 
3) Create a default adapter that no-ops on this call.