Added only_deleted() scope

This allows you to get only the rows that are marked as 'deleted' in the
database, without getting the rows that aren't marked as 'deleted'.
"$this->_temporary_only_deleted === TRUE" seems stupid.
Merge branch 'feature/only_deleted' of github.com:AKPWebDesign/codeigniter-base-model into AKPWebDesign-feature/only_deleted
Adjusting the soft delete conditions code - no need for multiple booleans
Adding tests for only_delete