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/memory/memory.py

@@ -18,6 +18,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 from __future__ import with_statement
+from sat.core.i18n import _
 
 import os.path
 import csv