Continued:

Continued:
- Added Receipts.compare() which does a comparison with result as
  Comparable.compareTo() would do.
- The difference here is that this method (like all its sisters and brothers)
  are truely null-safe.
- added private constructor to IncomeUtils, no instances shall be created from
  utilities classes
- also need to compare receipt item's category and receipt instance
- updated reference to Customers utility class

Signed-off-by: Roland Häder <roland@mxchange.org>