A Criterion is used to specify a condition on how to search through changesets.
Search changesets by A user id: Services_OpenStreetMap_Criterion('user', 12345)
A display/user name: Services_OpenStreetMap_Criterion('display_name', 'fredflintstone')
A bounding box: Services_OpenStreetMap_Criterion( 'bbox', -8.0590275, 52.9347449, -7.9966939, 52.9611999 )
For open changesets only: Services_OpenStreetMap_Criterion('open')
For closed changesets only: Services_OpenStreetMap_Criterion('closed')
For changesets created after a specific time: Services_OpenStreetMap_Criterion('time', '17/11/2011')
For changesets created during a specific timespan: Services_OpenStreetMap_Criterion('time', '17/11/2011', '29/11/2011')
Return the criterion type (closed, open, bbox, display_name, or user).
