fix bug returning default value for zero length elements
DicomDirectory class should not be disposable
move DicomDirectoryReaderObserver to Dicom.Media
fix menu bug in DICOM Dump utility when changing syntax to JPEG Lossless
Merge remote-tracking branch 'upstream/master'

Conflicts:
	ChangeLog.md
Merge branch 'master' into portable
Moved DicomDirectoryReaderObserver to Media