Toggle logging of ignored values in class method, not #define

This addresses the issue raised by @andrewsardone in krzysztofzablocki/KZPropertyMapper#10, using his proposed solution.

Users (CocoaPods users in particular) should now use the class method `+[KZPropertyMapper logIgnoredValues:]` to configure whether to log ignored values.

The previous method, via `#define KZPropertyMapperLogIgnoredValues`, will still work but is no longer exposed in the public interface.

Credit to @andrewsardone for the idea and for suggesting that the class method be placed in a Debug category,
to separate it from the class's main concern (that is, mapping stuff).