Fix #443
The root cause of the problem was inconsistent state of observer objects.
Because we have not unhook "dealloc" method, if XVim is disabled once and
a view is newly created and deallocated the method "removeObserver" is called
for a object which is not registered as a observer.

This has not been a problem in Xcode4 because Xcode4 deos not dealloc unused
view objects. Xcode5 does properly dealloc unused view objects which leads crashes.