Override getCalendarValue() for right output

Sorry, forgot to add this method to return the right value for this node
>-<
Add support for DT-Values + DT-Nodes

Now DateTimeNodes are avaible and there DateTime::getCalendarValue()
return something like
```
DTNODE:20131010T164525Z
```
or
```
DTNODE;VALUE=DATE:20131010
```
Simply add a PHPUnit Testcase...

...but useless for
me:
```Shell
user@host:~/playground/Dust-iCalendar/tests$ phpunit
UnitTest DateNodeTest.php
PHPUnit 3.6.12 by Sebastian
Bergmann.

.FF

Time: 0 seconds, Memory: 5.50Mb

There were 2
failures:

1) DateNodeTest::testStartICal
PHP Fatal error:  Call to
undefined function phpunit_mockobject_autoload() in
/usr/share/php/PHPUnit/Util/GlobalState.php on line 378
PHP Stack
trace:
PHP   1. {main}() /usr/bin/phpunit:0
PHP   2.
PHPUnit_TextUI_Command::main() /usr/bin/phpunit:46
PHP   3.
PHPUnit_TextUI_Command->run()
/usr/share/php/PHPUnit/TextUI/Command.php:130
PHP   4.
PHPUnit_TextUI_TestRunner->doRun()
/usr/share/php/PHPUnit/TextUI/Command.php:192
PHP   5.
PHPUnit_TextUI_ResultPrinter->printResult()
/usr/share/php/PHPUnit/TextUI/TestRunner.php:331
PHP   6.
PHPUnit_TextUI_ResultPrinter->printFailures()
/usr/share/php/PHPUnit/TextUI/ResultPrinter.php:164
PHP   7.
PHPUnit_TextUI_ResultPrinter->printDefects()
/usr/share/php/PHPUnit/TextUI/ResultPrinter.php:296
PHP   8.
PHPUnit_TextUI_ResultPrinter->printDefect()
/usr/share/php/PHPUnit/TextUI/ResultPrinter.php:226
PHP   9.
PHPUnit_TextUI_ResultPrinter->printDefectTrace()
/usr/share/php/PHPUnit/TextUI/ResultPrinter.php:239
PHP  10.
PHPUnit_Util_Filter::getFilteredStacktrace()
/usr/share/php/PHPUnit/TextUI/ResultPrinter.php:275
PHP  11.
PHPUnit_Util_GlobalState::phpunitFiles()
/usr/share/php/PHPUnit/Util/Filter.php:70
```
-__-
Forgot to add this with #4971ef9