track time spent alive and computes average healing over it

[pliesveld]

Modifies PlayerClassTracker to also calculate time player was spent alive. Enables capture of player_spawn events to do so. Calculates healing average over this recalculated time.

[dy-dx]

I can't comment on the actual code itself. but:

1. "Time Spent Alive" is very exciting and should be its own stat
2. "Average healing" does not need to be computed by the plugin, it is more flexible to have it done on the web side