Shorten "event_loop" to just "loop " in tests and example code

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I am tired of writing self.event_loop where self.loop would do.  Some code 
already uses just loop.  Let's use loop for all new code, and fix existing code 
opportunistically, one file at a time.

Note: I don't think we should change the official APIs.  Just local and 
instance variables used to hold the current event loop.

Original issue reported on code.google.com by gvanrossum@gmail.com on 25 Apr 2013 at 5:50