ENH: update DCMTK to use snapshot DCMTK-3.6.1_20150924

Created by: fedorov

The important improvements provided by the current DCMTK is the introduction of dcmiod, dcmfg and dcmseg classes that will be helpful in creating multiframe DICOM objects, such as segmentation and parametric map objects. This updated version also includes the feature to embed standard and private DICOM dictionaries. This makes it unnecessary to use DCMDICTPATH for loading dictionaries from external file, thus that functionality is removed.