Expose exports/ to non development environments

Remove dev-mode variable from config file as it was only used for
whether to register the exports/ route.
Return 404 Not Found if export file does not exist
Support multiple export directories in exports api

The exports api will look for a file in any of the export directories
given in the config file.  If the file is not found then it returns 404.
Add allowable types in config file and include it in the api
Set exports filename to <slugified title>.<file extension>
Redirect to latest version if exports url does not have a version
Merge pull request #12 from karenc/exports-api

Exports api