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
Retina button for HootSuite

Thanks again to vineviz, user at cocoaforge.com <http://forums.cocoaforge.com/viewtopic.php?t=26317&p=138487>
Fix responder in Unified layout

Make sure the UnifiedDisplayView responds to keyboard commands  after clicking on a link.
Change a term in French localization

Most applications, including Apple’s, use « Présentation » as menu title.
Workaround for issue #288

Subclass NSWindow, and apply techniques found at <http://lists.apple.com/archives/cocoa-dev/2008/Jun/msg01216.html> and <http://www.theregister.co.uk/2009/04/29/mac_secrets_framework/>
Silence warnings
Remove inoperative strings

As we handle this programmatically, don’t mislead translators.
Small housekeeping
Merge pull request #301 from barijaona/topic/MarkAllRead

Topic/mark all read
Merge pull request #302 from barijaona/topic/HootSuite

Retina button for HootSuite
Merge pull request #303 from barijaona/bugfixes

Fix responder in Unified layout
Merge pull request #304 from barijaona/topic/ShowToolbar

Solve issue #288 (localization of hide toolbar/show toolbar menu item)
Interim spanish localization sent by Juan Pablo Atienza Martinez
Upgrade version number to 3.0.0 RC 4