Languages support

Couple of problems here:

## CONST_Makefile
I tried but failed to create a Make command that only updates the pot file if one of the *.py or *.html files in the project changed, like so:
````
geoportailv3/locale/geoportailv3.pot: $(APP_HTML_FILES) $(APP_PYTHON_FILES)
   python setup.py extract_messages
````

So instead I just create a new make build target, which always runs when we do build 

## msgfmt 
I have problems with duplicate entries in the po files 
````
geoportailv3/locale/en/LC_MESSAGES/geoportailv3.po:7: duplicate message definition...
geoportailv3/locale/de/LC_MESSAGES/geoportailv3.po:8: ...this is the location of the first definition
geoportailv3/locale/en/LC_MESSAGES/geoportailv3.po:22: duplicate message definition...
geoportailv3/locale/de/LC_MESSAGES/geoportailv3.po:23: ...this is the location of the first definition
geoportailv3/locale/en/LC_MESSAGES/geoportailv3.po:26: duplicate message definition...
geoportailv3/locale/de/LC_MESSAGES/geoportailv3.po:27: ...this is the location of the first definition
geoportailv3/locale/en/LC_MESSAGES/geoportailv3.po:30: duplicate message definition...
geoportailv3/locale/de/LC_MESSAGES/geoportailv3.po:31: ...this is the location of the first definition
geoportailv3/locale/en/LC_MESSAGES/geoportailv3.po:34: duplicate message definition...
geoportailv3/locale/de/LC_MESSAGES/geoportailv3.po:35: ...this is the location of the first definition
geoportailv3/locale/en/LC_MESSAGES/geoportailv3.po:38: duplicate message definition...
geoportailv3/locale/de/LC_MESSAGES/geoportailv3.po:39: ...this is the location of the first definition
msgfmt: found 6 fatal errors
````

> @elemoine , as far as i can tell, python lingua only supports mako templates by making use of the babel-mako extractor package. so in effect, we still need Babel !?!
> 
> https://github.com/wichert/lingua#babel-extractors

IIUC, the Babel extractor for Mako comes with Mako itself, not with Babel. And if you have the Babel extractor for Mako then you should be able to use `pot-create` to extract messages. My understanding is that `pot-create` is what should be used when using lingua, whether we use babel extractors or not. In the future we may not use Babel at all, so the `setup.py extract_messages` will go away.