Issue 42 Patch

Fix for Issue: https://github.com/hgrecco/pint/issues/42

Fixed a dependency for manual non  'setup.py' install where 'pkg_resources.get_distribution('pint').version' would error out as it is not defined; __version__ is first assigned 'git describe' output, followed by get_distribution(...)' call and if that fails it defaults to “unknown”.