Add .text_content to events that have .content about chatexchange

[banksJeremy]

Something that would be interesting to think about is html2text, which could give us markdown text from the incoming messages. However, this would probably be less useful for most bots than simply ignoring the HTML tags, so I'll probably use the built-in HTMLParser.HTMLParser().unescape().

(edit: oh, that method doesn't strip HTML tags, so it isn't enough.)

[banksJeremy]

Passed my Travis here and your Travis here. Merging into default.