Update the crawler and export API.
Update the crawler and export API.

 - The crawl() method yields only the index documents,
   not a (_id, doc) tuple.
 - The export() and export_pymongo() function expect
   an iterable of index documents, not a crawler instance.
   The old API is still supported if the function is called
   without keyword arguments, in which case a DeprecationWarning
   is triggered.

csadorf/signac,csadorf/signac