add method to add and remove days from current timestamp incl. test
Fix NPE in Timestamp.after() and Timestamp.before() caused by Timestamp.toDate() with not matching date patterns
Refactoring: Timestamp.enrich*(..) => Timestamp.enrich(..)
Fixed timzone problem with Timestamp.applyDayDelta(int)
Refactoring: make Timestamp.enrich(Timestamp) more generic to update more attributes