Tightened up the code including:
- Hiding all classes and members that should not be externally exposed, mainly by making all but the BillingProcessor classes package protected, and data members private.
- Made the IBillingHandler into a public interface inside the BillingProcessor class.
- Updated the sample code & XML.
- Minor code formatting.
Note: Was not able to test with current build settings, so project files may need minor changes.