Small fixes, Xcode update and improvements

See commits for details.

An NSError should not be touched, unless you have determined that there was an error by checking the return value of a method. That’s why I added the `string == nil` to “-initWithFile:”.

BTW: Compiling against an SDK > 10.7 breaks compatibility with OS X <= 10.7 for parsing text formatting tags into an attributed string. 10.7 misses a few methods that the parser has reimplementations for when building against SDK <= 10.7. That is why I set the deployment target to 10.8.