Some metadata fields are not exported with notes and attachments

[dstillman]

Now that Item.toArray() is updated, export needs to handle embedded attachment notes (stored in the 'note' property of attachments) as well as URLs and access dates for attachments/notes, and possibly dateAdded and dateModified.

Since toArray() now just calls itself recursively on children, the export code should probably handle attachments like normal items (i.e. using any data toArray() makes available), since it's possible we'll add more metadata fields in the future.

[simon]

(In [753]) closes #336, Some metadata fields are not exported with notes and attachments
closes #165, verify import/export can carry all data for all fields and item types
closes #168, make sure MODS import works with files from external sources