Safety measure on GoogleReader

Cf Skripach crash logs at http://www.cocoaforge.com/viewtopic.php?f=18&t=26573&sid=38253d1334b27782d22d2f31daa258a9#p138418
Safety measure for transactions ; improve refresh indicator

After seeing http://www.cocoaforge.com/viewtopic.php?f=18&t=26578
Serialize uses of the jsonDecoder

As recommended by JSONKit documentation
Use the jsonDecoder itself for serialization
Fix crash

As described at http://forums.cocoaforge.com/viewtopic.php?t=26583&p=138447
Fix crash 2

Crash caused by simultaneous attempts to update count of unread articles
http://forums.cocoaforge.com/viewtopic.php?f=18&t=26573
Crash prevention

Should two processes attempt to simultaneously update unread count
Merge pull request #295 from barijaona/bugfixes

Prevent some crashes