Early detection of events without a player

There are a class of issues like #61 where __event.player is None__ so this PR tries to detect that condition higher in the call stack.

Author: cclauss

sc2reader/engine/engine.py

@@ -151,6 +151,7 @@ def run(self, replay):
         # the front of the line for immediate processing.
         while len(event_queue) > 0:
             event = event_queue.popleft()
+            assert event.player, 'Event with no player: {}'.format(event)
 
             if event.name == 'PluginExit':
                 # Remove the plugin and reset the handlers.