Made sync_translation_fields command respect the verbosity option.
Refactored sync_translation_fields for better readability.
Made it possible to run sync/update_translation_fields limited to a
single app.

The limiting only works with Django 1.7+. There is a new command line
option "--app" for both commands that takes an app label (last part of
name from INSTALLED_APPS with older apps or whatever is specified in
apps.py with newer). You can also directly pass an AppConfig instance
as a call_command argument, which is handy when executing the commands
as part of a post_migrate handler.
Replaced some % formatting with str.format() in commands code.

Only a few places benefit from the new syntax, but for consistency
changed all formatting in the commands.

Python 2.7+ compatible (2.6 would need argument numbers in bracers).
BaseCommand / handle without positional args is the new recommended
command style.

NoArgsCommand got deprecated in 1.8, see:
https://docs.djangoproject.com/en/dev/howto/custom-management-commands/
Removed a call to removed transaction.commit_unless_managed that
did nothing since 1.6.

See: https://docs.djangoproject.com/en/1.6/_modules/django/db/transaction/
Replaced optparse argument handling with the new argparse syntax.

See: https://docs.djangoproject.com/en/dev/releases/1.8/#extending-management-command-arguments-through-command-option-list