Message class, a new example, some other changes

Adds a `chatexchange.messages.Message` class. Events about messages will have a deduplicated `.message` reference to an instance. The Message will be updated with information from each of these events. If you try to access an attribute of a Message which is unknown, it will be fetched from the server.

Adds another example, a simple browser-based chat interface:

![screen shot 2014-05-06 at 3 16 42 am](https://cloud.githubusercontent.com/assets/953391/2886779/79f1d692-d4ee-11e3-8881-7c85336c0bfd.png)

Fixed some bugs. Upgrades BeautifulSoup. Documents a bit more.