Various changes for USHMM export from 26/06/14:

 - extract more details extent info
 - use brief_desc, collection_summary, and interview_summary as scope
   and content where appropriate
 - extract the funding note as a note
More arrangement out of did, and move USHMM provenance to custodhist
Capture some original USHMM info in other descriptive data fields
Treat document quantity/container like extent
Place container data in <container> element. Needs handling on the importer side. Closes #8
Add .gitignore