More logging
Fixed Open Reader requests

After discussion with FeedHQ and BazQux developers :
- changed handling of number of returned elements limitation ; this effectively reverts commit b31ec85992684526b9e9f8c9b1d4bab2035d8d05
- avoid passing global cookies, as Django may be tempted to use them
- no need anymore to pass tokens in the query string, as they are already passed as POST values
- we had a confusion between folder's "lastUpdate" (internally managed by Vienna) and "lastUpdateString" (provided by the Open Reader server)
More informative (and localizable) alert text