Merge pull request #14 from lbjay/master

pull 'ack' field into docs rather than use dbref
Merge pull request #15 from lbjay/master

session was still treating 'ack' as a referenced field
Merge pull request #16 from lbjay/master

add post-load munging to ranked bibstem labels
Merge pull request #17 from lbjay/master

don't use dbrefs for fulltext
set snapshot=True to ensure cursor doesn't iterate multiple times over same document