if the previous document does not exist, it must not stop the upload

Deleted documents returned an empty revs_info, throwing a TypeError on the script.

To test it, go to the storage directory then type: node synchro.js

"put" and "post" improved to update a document with its revision history
jio.put({"_id": "mydoc", "_revs": ["2-second", "1-first"]}, callback);
revs_info will be [{
  "rev": "2-second",
  "status": "available"
}, {
  "rev": "1-first",
  "status": "missing"
}]