Remove useless __str__ methods

Those objects have a much more useful __repr__
Add signals for creation/deletion/moving
Add other signals needed by livesync
Code style
Pass parent to deletion signals
Add after_process and before_retry signals
Remove unused uid_to_obj function
Contribution/SubContribution: __str__ => __repr__
i18n: make catalog compile work from setup.py
Missing 'domain' default
N_ -> ngettext in tpls to avoid confusion

Also fixed date formatting (localization)
Properly handle lazy strings
Add unit tests for i18n
Fix french translation according to changes