Update admin_manual/configuration/custom_user_backend.rst

improved example code to exactly match the required syntax.
Create background_jobs.rst

added description of background jobs base on information provided by the developer: http://mail.kde.org/pipermail/owncloud/2012-August/004759.html
Update admin_manual/configuration/background_jobs.rst
Update admin_manual/configuration/background_jobs.rst
Update admin_manual/configuration/background_jobs.rst
Merge pull request #51 from j-ed/master

improved example code to exactly match the required syntax.