Adding use_feature options to thruk.conf

The use_feature_histogram and use_feature_trends were not in thruk.conf and don't get set by any plugins.
Added use_feature_recurring_downtime option

The use_feature_recurring_downtime option allows for user to hide the recurring downtime links if they so choose.
Added use_service_description option

Allow user to have service description displayed instead of service display_name.
Merge pull request #161 from awiddersheim/master

Add more options dealing with what gets displayed and how