Implement absence listing report

See the "Absences" report from http://wiki.civicrm.org/confluence/display/HR/Absence+-+Manager+Perspective

Tasks:

T1. Implement the Report template with a unit-test (similar to the unit-tests in hrreport). Some notes are in the mockup. Additional notes:

• The table is essentially displaying a list of activities.
• The "absence type" column is the activity-type column.
• The results should always be filtered based on ACLs (ie you can only activities for contacts if you permission to see the contact)
• The only activity-types that can ever be used or shown are the activity types mentioned in civicrm_absence_type.
• The Qty is the sum of durations of the "Absence" activities. (e.g. "select sum(duration) from civicrm_activity where source_record_id = %d" to lookup the qty for a particular absence-request. To get them en masse, maybe "select source_record_id, sum(duration) from civicrm_activity where ... group by source_record_id)

T2. Register the ReportTemplate and ReportInstance classes (*.mgd.php)

T3. Ensure that a menu item appears in the "Absences" menu (like in mockup).