Some changes in DicomElemnt

Hello,
recently the DicomVR has changed to make PN, SH and LO to multi-string.
Until now this was not reflected in the corresponding classes DicomPersonName, DicomShortString, DicomLongString. I fixed this.
Additionally I did the following: 
- extended DicomPersonName with additional constructor 
- fixed the Last, First, Middle, Prefix, Suffix properties of DicomPersonName 
- added some tests for DicomPersonName 
Best regards, Lars