Make sure submodules submodules are upgraded as well when upgrading CGXP

As for now, when one upgrades CGXP using the procedure described in the doc here:
http://docs.camptocamp.net/c2cgeoportal/integrator/update_application.html#update-cgxp
only CGXP and its direct submodules are upgraded, not the submodules of the submodules.
Is there a reason for that?

This PR suggests to add a "--recursive" option to the submodules update.

By the way, would not
```
git pull
git submodule sync
git submodule update --init --recursive
``` 
be equivalent to
```
git pull
git submodule sync
git submodule update --init
git submodule foreach git submodule sync
git submodule foreach git submodule update --init
```
?
See http://docs.camptocamp.net/c2cgeoportal/integrator/update_application.html#update-the-application-code