Optimised for parallel downloading
Added /players method for Server REST interface

/root/servers/{pk}/players now returns a list of objects with
members name, score and duration.
Added /activity method for Server REST interface

/root/servers/{pk}/activity now returns a list of objects
representing the activity level (player_count and bot_count) for
a specific timestamp.

ServerViewSet.players also rewrote to use a serialiser.