This roughly parallels the way that blocking/eventsource.py and the pypi
"requests" modules work: the server can set the encoding (with
"Content-Type: text/event-stream; charset=utf-8"), and the EventSource
parser will decode accordingly. However eventsource_twisted.py *always*
returns unicode (on both py2/py3), even when the server hasn't set an
encoding. blocking/eventsource.py returns bytes (on py3, and str on py2)
when the server doesn't set an encoding.
In the future, eventsource_twisted.py should return bytes when the
server doesn't set an encoding.
eventsource_twisted.py includes an alternate approach that might be
necessary (a to_unicode() function instead of always using .decode), but
I won't be sure until enough of Twisted has been ported to allow the
EventSourceParser to be tested.
Also fix demo.py for python3.
