Allow session offerings to be counted only once

Sessions like small groups may have many offerings, but each student
only attends a single time.  This change allows users to mark these sessions
specially so they can be counted correctly in reporting.
added changes that from the re-tagged CodeIgniter 2.2.0 release.
Merge pull request #638 from stopfstedt/dirty_ci

added changes from the re-tagged CodeIgniter 2.2.0 release.
Merge pull request #635 from jrjohnson/473-smallgroupduration

Allow session offerings to be counted only once