Send email notifications

Addresses https://bugs.launchpad.net/oq-platform/+bug/1244288

Please note that I was not able to implement a unit test for this.

Mocking does not work as the django framework is doing something magical with django.core.email.

Django has its own way to address this issue, but you have to use its own test runner (atm we are using the standard test runner from nosetests).