IOPS fixes:

- Allow python to supply an event factory, for easier subclassing of C
types.
- Assign fd to event object before calling function
- Remove 'usecs' field from Timer. The relative timeout isn't very
  useful.

Change-Id: If9fe41dda6535952c3eadeeab2240e66f8941394
Reviewed-on: http://review.couchbase.org/29259
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>