Added ContactArchetypes.PURPOSE

git-svn-id: svn://svn.openvpms.org/openvpms/archetypes/branches/1.8-dev@6013 9e4fa352-aafb-0310-9071-df624684024b
Refactored ReminderRules.getContact() to use PurposeMatcher

git-svn-id: svn://svn.openvpms.org/openvpms/archetypes/branches/1.8-dev@6014 9e4fa352-aafb-0310-9071-df624684024b