Tests improvement

The test suite starts the Redis server on port 6399 and shuts it down it
process end
Added support for __iter__

Iterator by event returns the sorted sequence of all uuids matching the event

>>> bitmapist.mark_event('foo', 5)
>>> bitmapist.mark_event('foo', 50)
>>> bitmapist.mark_event('foo', 500)
>>> list(bitmapist.DayEvents('foo', y, m, d))
[5, 50, 500]
Added support for YearEvents and BitOpNot operation
Removed unused import