Fix a crash

Probable fix to <http://forums.cocoaforge.com/viewtopic.php?t=26592&p=138492>. A part of code was redundant and could, because of multithreading, make things out of sync regarding read/unread status.
Fix « mark all read » for OpenReader folders

- fix the array of articles to be marked read
- disambiguation of the name of a Database method
Add Undo for « Mark All Subscriptions As Read »

As discussed at http://forums.cocoaforge.com/viewtopic.php?t=26586
Merge pull request #301 from barijaona/topic/MarkAllRead

Topic/mark all read