Bump the version number to 0.5.0.

The major changes since 0.4.5 are as follows:
- Various cmake improvements, including CDash support
- The reader provides a vtkMedicalImageProperties object
- DICOM dictionary updated to the 2013 standard
- Resolution of private tags based on creator info
- niftitodicom supports RGB (for secondary capture only)
- native RLE codec (use DCMTK or GDCM for other codecs)
- API for vtkDICOMValue has been updated

The API changes to vtkDICOMValue are as follows:
- GetNumberOfValues returns a size_t value, not unsigned int,
  and values with encapsulated data can now be > 4GB in size
- Methods that used to specify a range of values via a start
  and end pointer, now use a start pointer and a size