fixes #30; follow MARC-to-JSON proposal

There was a critical bug in the serializer previously, that missed
repeated subfields. This commit updated `record.as_dict` so that it

* serializes a MARC record in JSON according to the format proposed under
  http://dilettantes.code4lib.org/blog/2010/09/a-proposal-to-serialize-marc-in-json/
Merge pull request #31 from miku/issue-30

fixes #30; follow MARC-to-JSON proposal