New feature: check for new versions.

fix #551
Early imports can be dangerous :p

see #551
Catch requests exceptions.

see #551
Display test results on stdout.
Adding missing requirement: requests.
Merge branch 'master' into feature/version_checker

Conflicts:
	modoboa/extensions/amavis/general_callbacks.py
	modoboa/extensions/amavis/views.py
Merge branch 'master' of github.com:tonioo/modoboa