Feature/backup task

We probably want to also merge this into the next 3.0.x version of OpenProject, so that it's easier for people to upgrade to 4.x

![automate all the backups](http://i.imgur.com/54ZPba4.jpg)

This [blog post](http://railsguides.net/do-you-test-your-rake-tasks/) inspired me to write the tests.


see: https://community.openproject.org/work_packages/16794