Added DT packages.
ユーザーが自由に作成したDT_Timeのサブクラスとも比較が出来るように変更。
テスト追加

Util_FormatException を今後どうするかは検討の必要性あり。
Use load.php for auto-load.
Use load.php for auto-load.
Refactoring.

- Converted DT_Time from abstract class to interface.
- Moved DT_Time implementation to new abstract class DT_AbstractTime.
- Created DT_TimeWrapper which is designed by Decorator Pattern.
Added new files.
Modified.
Added.
Created unit test for DT_Datetime and some other changes.

	modified:   src/DT/Date.php
	modified:   test/DT/DT_AbstractTimeTest.php
	modified:   test/DT/DT_DateTest.php
	new file:   test/DT/DT_DatetimeTest.php
	modified:   test/DT/DT_UtilTest.php
Modified some tests.
Modified testGet, testSet.
New unit test for DT_Timestamp.

Other modification:
  - Debug: DT_W3CDatetimeFormat
  - Fix:   tests for DT_Time::format
Added some tests and fixed bugs.

	modified:   src/DT/Datetime.php
	modified:   src/DT/FieldAdjuster.php
	modified:   src/DT/Timestamp.php
	modified:   src/DT/Util.php
	modified:   test/DT/DT_DatetimeTest.php
	modified:   test/DT/DT_TimestampTest.php
Modified some tests.
Created DT_HttpDateFormat, DT_SimpleFormat.
Modified some tests.
Modified require_once statements.
Formatted code.
Modified comments.
Replace assertType() to assertInstanceOf().
First commit for the tutorials of package DT.