Implement absence-calendar report

See the "Absences Calendar" 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 ordered by contact-name and date.
• Each cell corresponds to a date which may or may not have an activity.
• 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.

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

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