Courses don't have attachments right now
Add AttachableWithMetadata

includes metadata fields for attachments on editions
Add metadata save hooks and tests
Check that asset saving isn't duplicated
explicitly require attachment_with_metadata to fix test
Get values for metadata from asset API
Serialise spatial and subject fields properly into the DB
Instance variable conversion should not be in here; this is controller-level stuff.
Allow assignment of comma-separated strings to spatial and subject

I don't particularly like having this here, but I can't think of a better way right now.
Update update_asset call to match changed parameters
Update gds_api_adapters version to get update_asset API method
Update attachable_with_metadata tests for new API
Update gds-api-adapters version
DRY up assignment methods, and explain things with some comments
Merge pull request #7 from theodi/feature-64-asset-metadata

Asset metadata