core: i18n refactoring:

- _() is no more installed in __builtin__
    - instead, there is a new sat.core.i18n module
    - added D_() method for deferred translation
    - languageSwitch method allow to dynamically change translation language
    - import gettext is tested against ImportError, and dummy methods are used when not available (mainly useful for Libervia)

src/plugins/plugin_exp_command_export.py

@@ -17,6 +17,7 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+from sat.core.i18n import _
 from logging import debug, info, warning, error
 from twisted.words.protocols.jabber import jid
 from twisted.internet import reactor, protocol