Fixed: missing self. in call to _delegate.

Small formatting and code tweaks as well.
New: nib2cib gives a more helpful message when unsupported classes are encountered.

Previously users saw the generic "-[CPKeyedUnarchiver decodeObjectForKey:]: cannot decode object of class (NSFoo)" message, which was not particularly instructive.

Now the message is: "NSFoo objects are not supported by nib2cib."

Closes #1923