Issue 28 remove refs to event handler

[eyefodder]

This is a suggestion for updating the streaming notebook now that custom event handlers have been removed.

I originally thought to remove the cells, as the API is now pretty intuitive, and you cover the event sequence in the 'Working with streams" section. Then I figured it may still be useful to show the handling of these stream events for those who need to work with more events than the stream.text_stream lens.

I used @antsant's suggested fix for the original code in the example