SimpleITK: DICOM JPEG, NRRD, NIFTI (Windows only) (mlavik1#95)

* SimpleITK-based DICOM importer.
* Cleanup: Importer interfaces and factory.
* NRRD and NIFTI import (using SimpleITK)
* Added script for exporting .unitypackage
* Updated documentation.
* Don't use EditorDatasetImporter for RAW import.
* .gitattributes