Merge pull request #2 from adib/merge/karelia/dual-mode-arc

Allow compilation in both ARC and non-ARC modes.
Thanks!
MIT license
Appledoc
Annotate non-nil arguments
Add license file
#ifdef out -didChangeValueForKey: override when targeting 10.6+
Simpler unarchiving process
Gracefully handle invalid archives should clients wish
Merge commit '6febb6ac33462f57dbde30ed56c019dfdb328cf5'

* commit '6febb6ac33462f57dbde30ed56c019dfdb328cf5':
  Gracefully handle invalid archives should clients wish
  Simpler unarchiving process
  #ifdef out -didChangeValueForKey: override when targeting 10.6+
  Add license file
  Annotate non-nil arguments
  Appledoc
  MIT license