Implement setting and removing timespan attributes

Server.hs: Add postAttribute function, accessible as "POST /attributes",
for setting or removing a timespan attribute.

Spec.hs: Add tests for "POST /attributes" and modify firstTimespans to
take a list of expected attribute entities.

Signed-off-by: Stian Ellingsen    <stian@plaimi.net>
Reviewed-by:   Alexander Berntsen <alexander@plaimi.net>
Signed-off-by: Alexander Berntsen <alexander@plaimi.net>