NF: add copy of OrderedDict for Python 2.6 compat

DICOM will need OrderedDict; copy in the external module until we
require Python 2.7
Merge pull request #239 from matthew-brett/add-ordereddict

NF: add copy of OrderedDict for Python 2.6 compat

DICOM will need OrderedDict; copy in the external module until we require
Python 2.7