Fix entity-escaping in hangout session notices

Fixes issue #289. Google hangout (probably erroneously)
double-entity-escapes text in displayed notices. To work around this,
convert apostrophes, quotes, ampersands, and angle brackets to innocuous
unicode alternatives.

displayNotice docs:
https://developers.google.com/+/hangouts/api/gapi.hangout.layout#gapi.hangout.layout.displayNotice