Add note to NEWS.txt about PICARD-205 being fixed.
Add a update_countries command to setup.py

It retrieves countries.pot from transifex and uses it to generated picard/countries.py
Add po/countries/countries.pot to git ignore list

We don't want this one to be committed to git repo, it is not created
by parsing Picard code.
Add generated picard/countries.py
Replace the release countries in const.py with import from countries.py.
Use ungettext_countries() for translating country names.
Add po/countries/*.po from transifex