Add some CaseTest.

CaseTest
	NextDateProblem_02_15_2001_02162001_Case16
	NextDateProblem_02_28_2004_02292004_Case17
	NextDateProblem_02_28_2001_03012001_Case18
	NextDateProblem_02_29_2004_03012004_Case19
	NextDateProblem_02_29_2001_InvalidInputDate_Case20
	NextDateProblem_02_30_2001_InvalidInputDate_Case21toCase22

Changes to be committed:
	modified:   HW1/HW1 ClassLibrary/HW1_ClassLibrary.cs
	modified:   HW1/HW1 UnitTest/UnitTest1.cs