Add a plugin for static files builds

And use properties for dependent values defined during __init__
Remove pre-existing .mo file before build

Make sure we don't use a stale one!
Only use i18n if locale is set

This means we don't get error messages in case xgettext is not installed
while locale is not defined.

Extra useful since the newer (1.6) version of django will raise an error
in case msgfmt is not found (in compilemessages)