Use alpha Humbug receive-messages API rather than scraping.

Hi Thomas,

We love your enthusiasm. We were hoping to spend a bit more time polishing the receive-messages API (and we still will), but given the lengths you're going to to scrape the messages today, it seemed better for you to have our API in our unsupported may-change form than not at all :)

The attached PR should make humbot work using this API; I don't believe any other changes are necessary.

This requires python-humbug 0.1.4~hackerschool, available at
https://humbughq.com/dist/api/python-humbug_0.1.4~hackerschool.tar.gz

- Waseem, for all of us here at Humbug