PICARD-123: actually use default drive on Mac

This doesn't implement an automatic list of available drives on Mac,
but adds the libdiscid default drive to the list of drive candidates.

This should make disc lookups available on Mac without specifying
the drive (as /dev/rdisk*) in the settings.

Note that setting things like "1" or "2" as drives should also work
with a recent libdiscid. (>= 0.6.0).
use only one definition of get_cdrom_drives()
remove generic try-except

If we do want to catch generic unexpected tracebacks,
this shouldn't be done here.

Note that `import discid` might raise OSError
when the python module is found, but not libdiscid itself.
We might want to catch that explicitely,
but that is supposed to be prevented by whoever made `discid` available.
refactor cdrom code a bit
update NEWS for 123
Merge branch 'master' into always_default_drive

Conflicts:
	picard/util/cdrom.py
Set up Transifex to pull the country name translations.
Build picard-countries.po for enabled languages.
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.
Build, install and use all available locales

Drop the use of UI_LANGUAGES in setup.py which was restricting available translations,
even though translation was available from transifex.

Locales not in UI_LANGUAGES cannot be selected from Picard Advanced UI option,
but will be used if it is set to system default.
Add command get_po_files to setup.py

It runs tx pull with needed options.
Translations with less than 5% translated are ignored.
Use a constant for common options of tx pull
Update countries.pot parser since format changed (iso_code -> iso.code)

It changed in https://bitbucket.org/metabrainz/musicbrainz-server/pull-request/426/mbs-6373-only-translate-and-extract-area/diff
Warn if no country code was found, ie. when file format changed.
Regenerate countries.py from updated countries.pot