New feature: check for new versions.

fix #551
Early imports can be dangerous :p

see #551
Catch requests exceptions.

see #551
Regroup all queries in one place.

see #589
Few fixes in the postgres backend.

see #589
Adding some comments.

see #589
Merge branch '1.1.x' into refactoring/amavis_queries
Merge branch '1.1.x'
- Fix typo.
Merge pull request #593 from aanriot/master

Fix typo
- Missing ";" in doc/integration/web_servers.rst.
bootstrap-datepicker.js -> bootstrap-datetimepicker.js
Merge pull request #595 from aanriot/master

Typo in doc/integration/web_servers.rst
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
Merge branch 'master' into feature/radicale

Conflicts:
	modoboa/extensions/amavis/app_settings.py
	tests/install_from_scratch.py