Small fixes, Xcode update and improvements

See commits for details.

The error should not be touched, unless you know there was one 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.