Updates UID.py and _UID_dict.py (no change) to common py2/3 code
Modernizes (py2/3 common code) main pydicom files up to (not incl) tag.py alphabetically.

Files were run through python-modernize (uses six library) with
python-modernize <filename> -wx libmodernize.fixes.fix_import,
i.e. did not force absolute import __future__ as we already use absolute everywhere

Full test suite still passes under py26, py27.
Merge branch 'dev' of https://github.com/darcymason/pydicom into dev
more pythonic CT pixel data test
Merge branch 'dev' of https://github.com/jrkerns/pydicom into jrkerns-dev
Merge branch 'jrkerns-dev' into dev