CPDatePicker fixes, optimizations, tweaks...

- CPImageInBundle() can't be used from a ThemeDescriptor. Replaced with PatternImage().
- DRY! Put clock image sizes in variables so they can be maintained in one place.
- When displayed from the theme showcase, the hand images in HandLayer -setImage: were still _CPCibCustomResources, not sure why. This condition is now explicitly handled by converting to the referenced image.
- Fixed a bug in _CPDatePickerClock -setEnabled where the sublayers would not update unless the event loop was pumped.
- Check for unchanged value in setters.
- Various code optimizations.
- Removed unnecessary method comments.
- Fixed typo in demo app.
Fixed: in some cases nib2cib would write to stdout, which wasn't captured by XcodeCapp.

Previously, only stderr was checked for response text. In some edge cases it would write an error message to stdout. In addition, the default error message provided when the return status was non-zero and the output was empty was implemented incorrectly.

Now all output from nib2cib is correctly captured.

Also updated help to indicate how to enable debug logging.