Add prefix to ensure ref_ids are valid in EAD
EAD export: only add <address> tag if there is an address
EAD export: revise extent mapping and handle empty component IDs
EAD import: map physdesc only to Extent, not to Notes
EAD export: don't insert <ref/> when reference_text is empty
Merge pull request #15 from anarchivist/ead-export-tweaks

EAD import/export tweaks (thanks to Mark Custer for feedback)