Add unittest2 import exception

A proposal to throw an error if unittest2 is not installed.
Merge pull request #159 from jrkerns/patch-1

Add unittest2 import exception
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
Modernizes tag.py common py2/3 code.  Fixes relative import in _UID_dict